diff --git a/Dockerfile b/Dockerfile
index dc3f73f4..8c028681 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -4,6 +4,8 @@ FROM nginx:alpine
 # PERF-019: Only copy necessary web files (exclude .git, desktop-app, wiki, etc.)
 COPY index.html /usr/share/nginx/html/
 COPY workspace-storage.js /usr/share/nginx/html/
+COPY visual-settings-adapters.js /usr/share/nginx/html/
+COPY visual-settings-store.js /usr/share/nginx/html/
 COPY script.js /usr/share/nginx/html/
 COPY styles.css /usr/share/nginx/html/
 COPY sw.js /usr/share/nginx/html/
@@ -12,6 +14,8 @@ COPY manifest.json /usr/share/nginx/html/
 COPY robots.txt /usr/share/nginx/html/
 COPY sitemap.xml /usr/share/nginx/html/
 COPY assets/ /usr/share/nginx/html/assets/
+COPY seo/locales.mjs /usr/share/nginx/html/seo/
+COPY seo/welcome-content.mjs /usr/share/nginx/html/seo/
 
 # Create a custom nginx configuration with compression and security
 # PERF-020: Added gzip compression for text-based assets
diff --git a/desktop-app/prepare.js b/desktop-app/prepare.js
index b265bf44..2027b792 100644
--- a/desktop-app/prepare.js
+++ b/desktop-app/prepare.js
@@ -49,6 +49,15 @@ console.log("✓ Copied script.js → resources/js/script.js");
 fs.copyFileSync(path.join(ROOT_DIR, "workspace-storage.js"), path.join(jsDest, "workspace-storage.js"));
 console.log("✓ Copied workspace-storage.js → resources/js/workspace-storage.js");
 
+if (fs.existsSync(path.join(ROOT_DIR, "visual-settings-store.js"))) {
+  fs.copyFileSync(path.join(ROOT_DIR, "visual-settings-store.js"), path.join(jsDest, "visual-settings-store.js"));
+  console.log("✓ Copied visual-settings-store.js → resources/js/visual-settings-store.js");
+}
+if (fs.existsSync(path.join(ROOT_DIR, "visual-settings-adapters.js"))) {
+  fs.copyFileSync(path.join(ROOT_DIR, "visual-settings-adapters.js"), path.join(jsDest, "visual-settings-adapters.js"));
+  console.log("✓ Copied visual-settings-adapters.js → resources/js/visual-settings-adapters.js");
+}
+
 fs.copyFileSync(path.join(ROOT_DIR, "preview-worker.js"), path.join(jsDest, "preview-worker.js"));
 console.log("Copied preview-worker.js to resources/js/preview-worker.js");
 
@@ -262,9 +271,9 @@ async function prepareOfflineDependencies() {
       hash: null
     },
     {
-      url: "https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js",
-      dest: path.join(LIBS_DIR, "tex-mml-chtml.min.js"),
-      hash: null
+      url: "https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js",
+      dest: path.join(LIBS_DIR, "tex-mml-chtml.js"),
+      hash: "sha384-2f5bAKjuFIbbQ+0O9aSu5W1OyLA4q80QrDEvjXDwJexjU9VqoJhMgw19pYVpSJab"
     },
     {
       url: "https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.5.1/jspdf.umd.min.js",
@@ -305,6 +314,12 @@ async function prepareOfflineDependencies() {
   // Wait for all downloads and cryptographic validations to finish
   try {
     await Promise.all(downloads);
+    // Keep backwards-compatible copy for tex-mml-chtml.min.js
+    const mathjaxDest = path.join(LIBS_DIR, "tex-mml-chtml.js");
+    const mathjaxMinDest = path.join(LIBS_DIR, "tex-mml-chtml.min.js");
+    if (fs.existsSync(mathjaxDest)) {
+      fs.copyFileSync(mathjaxDest, mathjaxMinDest);
+    }
     console.log("✓ All offline libraries successfully downloaded and cryptographically validated.");
   } catch (err) {
     console.error("✗ Critical Security Error: Dependency integrity check failed!", err.message);
@@ -320,6 +335,8 @@ async function prepareOfflineDependencies() {
   html = html.replace(/href="assets\//g, 'href="/assets/');
   html = html.replace(/href="styles\.css"/g, 'href="/styles.css"');
   html = html.replace(/href="workspace-storage\.js"/g, 'href="/js/workspace-storage.js"');
+  html = html.replace(/href="visual-settings-adapters\.js"/g, 'href="/js/visual-settings-adapters.js"');
+  html = html.replace(/href="visual-settings-store\.js"/g, 'href="/js/visual-settings-store.js"');
   html = html.replace(/href="script\.js"/g, 'href="/js/script.js"');
   
   // PERF-034: Strip web-specific SEO tags, canonical, hreflang, preconnect, manifest and JSON-LD structured data for desktop build
@@ -333,8 +350,18 @@ async function prepareOfflineDependencies() {
 
   // Inject Neutralino script tags
   html = html.replace(
-    /<script\s+src="workspace-storage\.js"[^>]*><\/script>\s*<script\s+src="script\.js"[^>]*><\/script>/i,
-    '<script src="/js/neutralino.js"></script>\n    <script src="/js/main.js"></script>\n    <script src="/js/workspace-storage.js"></script>\n    <script src="/js/script.js"></script>',
+    /<script\s+src="workspace-storage\.js"[^>]*><\/script>(?:\s*<script\s+src="visual-settings-adapters\.js"[^>]*><\/script>)?(?:\s*<script\s+src="visual-settings-store\.js"[^>]*><\/script>)?\s*<script\s+src="script\.js"[^>]*><\/script>/i,
+    (match) => {
+      let tags = '<script src="/js/neutralino.js"></script>\n    <script src="/js/main.js"></script>\n    <script src="/js/workspace-storage.js"></script>';
+      if (match.includes('visual-settings-adapters.js')) {
+        tags += '\n    <script src="/js/visual-settings-adapters.js"></script>';
+      }
+      if (match.includes('visual-settings-store.js')) {
+        tags += '\n    <script src="/js/visual-settings-store.js"></script>';
+      }
+      tags += '\n    <script src="/js/script.js"></script>';
+      return tags;
+    }
   );
 
   // Inject app-info element
diff --git a/desktop-app/resources/index.html b/desktop-app/resources/index.html
index e4192e2f..f101008c 100644
--- a/desktop-app/resources/index.html
+++ b/desktop-app/resources/index.html
@@ -7,6 +7,8 @@
     <!-- PERF-015: Preload critical-path resources for faster discovery -->
     <link rel="preload" href="/styles.css" as="style">
     <link rel="preload" href="/js/workspace-storage.js" as="script">
+    <link rel="preload" href="/js/visual-settings-adapters.js" as="script">
+    <link rel="preload" href="/js/visual-settings-store.js" as="script">
     <link rel="preload" href="/js/script.js" as="script">
 
     <!-- PWA Web Manifest -->
@@ -121,6 +123,10 @@ <h1 class="h4 mb-0">Markdown Viewer<span class="visually-hidden"> - Online Markd
                         <i class="lucide lucide-info" aria-hidden="true"></i>
                     </button>
 
+                    <button id="visual-settings-button" class="header-icon-command" type="button" title="Visual library settings" aria-label="Open visual library settings">
+                        <i class="lucide lucide-layers-3" aria-hidden="true"></i>
+                    </button>
+
                     <div class="dropdown settings-dropdown">
                         <button class="header-icon-command" type="button" id="workspaceSettingsDropdown" data-bs-toggle="dropdown" data-bs-auto-close="outside" aria-expanded="false" title="Workspace settings (Ctrl/Cmd+,)" aria-label="Open workspace settings" aria-keyshortcuts="Control+, Meta+,">
                             <i class="lucide lucide-settings" aria-hidden="true"></i>
@@ -132,6 +138,9 @@ <h1 class="h4 mb-0">Markdown Viewer<span class="visually-hidden"> - Online Markd
                             <button id="theme-toggle" class="settings-menu-item app-menu-item settings-menu-item--toggle" type="button" aria-pressed="false" title="Toggle appearance">
                                 <i id="theme-toggle-icon" class="lucide lucide-sun" aria-hidden="true"></i><span class="app-menu-copy"><span class="app-menu-label">Appearance</span><span id="theme-toggle-description" class="app-menu-description">Switch to dark appearance</span></span><span class="settings-switch themed-settings-switch" aria-hidden="true"><span><i id="theme-switch-icon" class="lucide lucide-sun settings-switch-icon"></i></span></span>
                             </button>
+                            <button id="menu-visual-settings-button" class="settings-menu-item app-menu-item" type="button" title="Visual library settings">
+                                <i class="lucide lucide-layers-3" aria-hidden="true"></i><span class="app-menu-copy"><span class="app-menu-label">Visual Settings</span><span class="app-menu-description">Themes, fonts, and styles for libraries</span></span>
+                            </button>
                             <div class="dropdown dropstart settings-language-dropdown">
                                 <button class="settings-menu-item app-menu-item dropdown-toggle" type="button" id="languageDropdown" data-bs-toggle="dropdown" data-bs-auto-close="outside" aria-expanded="false" title="Switch language">
                                     <i class="lucide lucide-languages" aria-hidden="true"></i><span class="app-menu-copy"><span id="current-lang-label" class="app-menu-label">English</span><span class="app-menu-description">Application language</span></span>
@@ -316,6 +325,9 @@ <h3 class="mobile-menu-group-label">Actions</h3>
                                         <button id="mobile-private-mode-toggle" class="mobile-menu-item mobile-settings-item mobile-settings-toggle" type="button" aria-pressed="false" title="Turn private mode on">
                                             <span class="mobile-menu-item-label"><i class="lucide lucide-hat-glasses" aria-hidden="true"></i><span>Private mode</span></span><span class="settings-switch themed-settings-switch" aria-hidden="true"><span></span></span>
                                         </button>
+                                        <button id="mobile-visual-settings-button" class="mobile-menu-item mobile-settings-item" type="button" title="Visual library settings">
+                                            <span class="mobile-menu-item-label"><i class="lucide lucide-layers-3" aria-hidden="true"></i><span>Visual Settings</span></span>
+                                        </button>
                                         <button id="mobile-storage-settings-button" class="mobile-menu-item mobile-settings-item" type="button" title="Storage and backup">
                                             <span class="mobile-menu-item-label"><i class="lucide lucide-box" aria-hidden="true"></i><span>Storage and Backup</span></span>
                                         </button>
@@ -559,6 +571,83 @@ <h2 id="document-sidebar-title">Explorer</h2>
         </div>
         <input id="storage-backup-file-input" type="file" accept=".zip,application/zip" hidden>
 
+        <!-- Visual Library Settings Modal -->
+        <div id="visual-settings-modal" class="reset-modal-overlay modal-overlay" role="dialog" aria-modal="true" aria-labelledby="visual-settings-title" aria-hidden="true" style="display:none;">
+          <div class="reset-modal-box reset-modal-box--wide app-modal-box modal-box visual-settings-box">
+            <div class="modal-header">
+              <p id="visual-settings-title" class="reset-modal-message">Visual Library Settings</p>
+              <button type="button" class="modal-close-btn" id="visual-settings-close-icon" aria-label="Close visual library settings">
+                <i class="lucide lucide-x" aria-hidden="true"></i>
+              </button>
+            </div>
+            <div class="modal-body visual-settings-body">
+              <p class="visual-setting-desc mb-2">Configure default themes, fonts, and rendering styles for preview visual libraries.</p>
+
+              <!-- highlight.js theme -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-hljs-theme" class="visual-setting-label">
+                  <i class="lucide lucide-code-2" aria-hidden="true"></i> Syntax Highlighting Theme (highlight.js)
+                </label>
+                <select id="visual-setting-hljs-theme" class="form-select visual-setting-select" aria-describedby="visual-setting-hljs-desc">
+                </select>
+                <p id="visual-setting-hljs-desc" class="visual-setting-desc">Color palette for highlighted code blocks in the preview.</p>
+              </div>
+
+              <!-- MathJax math font -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-math-font" class="visual-setting-label">
+                  <i class="lucide lucide-spline" aria-hidden="true"></i> Math Font (MathJax)
+                </label>
+                <select id="visual-setting-math-font" class="form-select visual-setting-select" aria-describedby="visual-setting-math-desc">
+                </select>
+                <p id="visual-setting-math-desc" class="visual-setting-desc">Font family used for rendering LaTeX mathematical notation.</p>
+              </div>
+
+              <!-- Mermaid theme -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-mermaid-theme" class="visual-setting-label">
+                  <i class="lucide lucide-workflow" aria-hidden="true"></i> Diagram Theme (Mermaid)
+                </label>
+                <select id="visual-setting-mermaid-theme" class="form-select visual-setting-select" aria-describedby="visual-setting-mermaid-theme-desc">
+                </select>
+                <p id="visual-setting-mermaid-theme-desc" class="visual-setting-desc">Color palette and appearance for Mermaid diagrams.</p>
+              </div>
+
+              <!-- Mermaid look -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-mermaid-look" class="visual-setting-label">
+                  <i class="lucide lucide-square-pen" aria-hidden="true"></i> Diagram Drawing Style (Mermaid Look)
+                </label>
+                <select id="visual-setting-mermaid-look" class="form-select visual-setting-select" aria-describedby="visual-setting-mermaid-look-desc">
+                </select>
+                <p id="visual-setting-mermaid-look-desc" class="visual-setting-desc">Geometric drawing style (Classic or Hand Drawn sketch).</p>
+              </div>
+
+              <!-- JoyPixels emoji skin tone -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-emoji-skin-tone" class="visual-setting-label">
+                  <i class="lucide lucide-smile" aria-hidden="true"></i> Emoji Skin Tone (JoyPixels)
+                </label>
+                <select id="visual-setting-emoji-skin-tone" class="form-select visual-setting-select" aria-describedby="visual-setting-emoji-skin-tone-desc">
+                </select>
+                <p id="visual-setting-emoji-skin-tone-desc" class="visual-setting-desc">Default Fitzpatrick skin tone for supported emoji shortcodes.</p>
+              </div>
+            </div>
+            <div class="reset-modal-actions visual-settings-actions">
+              <div class="visual-settings-actions-left">
+                <button type="button" class="reset-modal-btn reset-modal-cancel" id="visual-settings-reset-btn" title="Reset all visual settings to defaults">
+                  <i class="lucide lucide-refresh-cw me-1" aria-hidden="true"></i> Reset to defaults
+                </button>
+              </div>
+              <div class="visual-settings-actions-right">
+                <button type="button" class="reset-modal-btn storage-primary-action" id="visual-settings-close-btn">
+                  Close
+                </button>
+              </div>
+            </div>
+          </div>
+        </div>
+
         <!-- Trash Modal -->
         <div id="trash-modal" class="reset-modal-overlay modal-overlay" role="dialog" aria-modal="true" aria-labelledby="trash-modal-title" aria-describedby="trash-modal-description" aria-hidden="true" style="display:none;">
           <div class="reset-modal-box reset-modal-box--wide app-modal-box modal-box trash-modal-box">
@@ -2214,6 +2303,8 @@ <h3>No comments yet</h3>
     <script src="/js/neutralino.js"></script>
     <script src="/js/main.js"></script>
     <script src="/js/workspace-storage.js"></script>
+    <script src="/js/visual-settings-adapters.js"></script>
+    <script src="/js/visual-settings-store.js"></script>
     <script src="/js/script.js"></script>
     <!-- Screen reader dynamic accessibility announcer -->
     <div id="app-accessibility-announcer" class="visually-hidden" aria-live="polite" aria-atomic="true"></div>
diff --git a/desktop-app/resources/js/script.js b/desktop-app/resources/js/script.js
index 4c25a960..b775309c 100644
--- a/desktop-app/resources/js/script.js
+++ b/desktop-app/resources/js/script.js
@@ -111,6 +111,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // PERF-002: Lazy script loader for optional heavy libraries
   const CDN_INTEGRITY = Object.freeze({
     'https://cdn.jsdelivr.net/npm/mermaid@11.15.0/dist/mermaid.min.js': 'sha384-yQ4mmBBT+vhTAwjFH0toJXNYJ6O4usWnt6EPIdWwrRvx2V/n5lXuDZQwQFeSFydF',
+    'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js': 'sha384-2f5bAKjuFIbbQ+0O9aSu5W1OyLA4q80QrDEvjXDwJexjU9VqoJhMgw19pYVpSJab',
     'https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js': 'sha384-M5jmNxKC9EVnuqeMwRHvFuYUE8Hhp0TgBruj/GZRkYtiMrCRgH7yvv5KY+Owi7TW',
     'https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.5.1/jspdf.umd.min.js': 'sha384-JcnsjUPPylna1s1fvi1u12X5qjY5OL56iySh75FdtrwhO/SWXgMjoVqcKyIIWOLk',
     'https://cdnjs.cloudflare.com/ajax/libs/html2canvas/1.4.1/html2canvas.min.js': 'sha384-ZZ1pncU3bQe8y31yfZdMFdSpttDoPmOZg2wguVK9almUodir1PghgT0eY7Mrty8H',
@@ -188,7 +189,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // CDN URLs for lazy-loaded libraries
   const CDN = {
     mermaid: 'https://cdn.jsdelivr.net/npm/mermaid@11.15.0/dist/mermaid.min.js',
-    mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js',
+    mathjax: 'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js',
     jspdf: 'https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.5.1/jspdf.umd.min.js',
     html2canvas: 'https://cdnjs.cloudflare.com/ajax/libs/html2canvas/1.4.1/html2canvas.min.js',
     pako: 'https://cdnjs.cloudflare.com/ajax/libs/pako/2.1.0/pako.min.js',
@@ -211,7 +212,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // Resolve local paths for desktop (Neutralinojs) offline support
   if (typeof Neutralino !== 'undefined') {
     CDN.mermaid = '/libs/mermaid.min.js';
-    CDN.mathjax = '/libs/tex-mml-chtml.min.js';
+    CDN.mathjax = '/libs/tex-mml-chtml.js';
     CDN.jspdf = '/libs/jspdf.umd.min.js';
     CDN.html2canvas = '/libs/html2canvas.min.js';
     CDN.pako = '/libs/pako.min.js';
@@ -943,6 +944,18 @@ document.addEventListener("DOMContentLoaded", async function () {
   const aboutVersion = document.getElementById("about-version");
   const aboutReleaseNotes = document.getElementById("about-release-notes");
   const privateModeToggle = document.getElementById("private-mode-toggle");
+  const visualSettingsButton = document.getElementById("visual-settings-button");
+  const menuVisualSettingsButton = document.getElementById("menu-visual-settings-button");
+  const mobileVisualSettingsButton = document.getElementById("mobile-visual-settings-button");
+  const visualSettingsModal = document.getElementById("visual-settings-modal");
+  const visualSettingsCloseBtn = document.getElementById("visual-settings-close-btn");
+  const visualSettingsCloseIcon = document.getElementById("visual-settings-close-icon");
+  const visualSettingsResetBtn = document.getElementById("visual-settings-reset-btn");
+  const selectHljsTheme = document.getElementById("visual-setting-hljs-theme");
+  const selectMathFont = document.getElementById("visual-setting-math-font");
+  const selectMermaidTheme = document.getElementById("visual-setting-mermaid-theme");
+  const selectMermaidLook = document.getElementById("visual-setting-mermaid-look");
+  const selectEmojiSkinTone = document.getElementById("visual-setting-emoji-skin-tone");
   const storageSettingsButton = document.getElementById("storage-settings-button");
   const trashSettingsButton = document.getElementById("trash-settings-button");
   const storageSettingsModal = document.getElementById("storage-settings-modal");
@@ -2237,22 +2250,28 @@ document.addEventListener("DOMContentLoaded", async function () {
   applyDirectionToContent(initialDirection);
   updateDirectionToggleUI(initialDirection);
 
-  // Track last Mermaid theme to avoid redundant re-initialization (PERF-005)
+  // Track last Mermaid theme and look to avoid redundant re-initialization (PERF-005)
   let _lastMermaidTheme = null;
   let _mermaidThemeReinitTimeout = null;
   let _themeTransitionTimeout = null;
   const initMermaid = (forceReinit, themeOverride) => {
     if (typeof mermaid === 'undefined') return; // PERF-002: Not loaded yet
     const currentTheme = themeOverride || document.documentElement.getAttribute("data-theme");
-    const mermaidTheme = currentTheme === "dark" ? "dark" : "default";
+    const config = window.visualSettingsAdapters
+      ? window.visualSettingsAdapters.getMermaidConfig(currentTheme)
+      : { theme: currentTheme === "dark" ? "dark" : "default", look: "classic" };
+    const mermaidTheme = config.theme;
+    const mermaidLook = config.look || "classic";
     
-    // Skip re-initialization if theme hasn't changed (PERF-005)
-    if (!forceReinit && _lastMermaidTheme === mermaidTheme) return;
-    _lastMermaidTheme = mermaidTheme;
+    // Skip re-initialization if theme & look haven't changed (PERF-005)
+    const configKey = `${mermaidTheme}:${mermaidLook}`;
+    if (!forceReinit && _lastMermaidTheme === configKey) return;
+    _lastMermaidTheme = configKey;
     
     mermaid.initialize({
       startOnLoad: false,
       theme: mermaidTheme,
+      look: mermaidLook,
       securityLevel: 'strict',
       flowchart: { useMaxWidth: true, htmlLabels: true },
       fontSize: 16,
@@ -4108,7 +4127,18 @@ document.addEventListener("DOMContentLoaded", async function () {
 
   function configureMathJax() {
     if (window.MathJax && typeof MathJax.typesetPromise === 'function') return;
-    if (window.MathJax && MathJax.loader && MathJax.tex) return;
+    if (window.MathJax && MathJax.loader && MathJax.tex) {
+      if (window.MathJax.chtml && typeof window.MathJax.chtml.font === 'string') {
+        delete window.MathJax.chtml.font;
+      }
+      return;
+    }
+    const initialFont = window.visualSettingsAdapters
+      ? window.visualSettingsAdapters.getCurrentMathFont()
+      : 'mathjax-modern';
+    if (window.MathJax && window.MathJax.chtml && typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
     window.MathJax = {
       startup: {
         typeset: false
@@ -4116,6 +4146,9 @@ document.addEventListener("DOMContentLoaded", async function () {
       options: {
         a11y: { inTabOrder: false }
       },
+      output: {
+        font: initialFont
+      },
       tex: {
         inlineMath: [['$', '$'], ['\\(', '\\)']],
         displayMath: [['$$', '$$'], ['\\[', '\\]']],
@@ -4133,7 +4166,15 @@ document.addEventListener("DOMContentLoaded", async function () {
 
     configureMathJax();
     if (!mathJaxLoadPromise) {
-      mathJaxLoadPromise = loadScript(CDN.mathjax)
+      const initialFont = window.visualSettingsAdapters
+        ? window.visualSettingsAdapters.getCurrentMathFont()
+        : 'mathjax-modern';
+      const fontPreloadPromise = (initialFont && initialFont !== 'mathjax-modern' && window.visualSettingsAdapters)
+        ? window.visualSettingsAdapters.applyMathFont(initialFont)
+        : Promise.resolve();
+
+      mathJaxLoadPromise = fontPreloadPromise
+        .then(function() { return loadScript(CDN.mathjax); })
         .then(function() {
           return MathJax.startup && MathJax.startup.promise
             ? MathJax.startup.promise
@@ -15014,6 +15055,7 @@ ${selector} .arrowheadPath {
   }
 
   function renderMarkdown(options) {
+    window.renderMarkdown = renderMarkdown;
     stopActiveAbcPlayback();
     options = options || {};
     const rawVal = markdownEditor.value;
@@ -17058,7 +17100,10 @@ ${selector} .arrowheadPath {
       
       while ((match = emojiRegex.exec(text)) !== null) {
         const shortcode = match[1];
-        const emoji = joypixels.shortnameToUnicode(`:${shortcode}:`);
+        const effectiveShortcode = (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.resolveEmojiShortcode === 'function')
+          ? window.visualSettingsAdapters.resolveEmojiShortcode(shortcode)
+          : shortcode;
+        const emoji = joypixels.shortnameToUnicode(`:${effectiveShortcode}:`);
         
         if (emoji !== `:${shortcode}:`) { // If conversion was successful
           hasEmoji = true;
@@ -24031,6 +24076,13 @@ ${selector} .arrowheadPath {
     });
   }
 
+  if (mobileVisualSettingsButton) {
+    mobileVisualSettingsButton.addEventListener('click', function() {
+      closeMobileMenu();
+      openVisualSettingsModal(mobileVisualSettingsButton);
+    });
+  }
+
   if (mobileStorageSettingsButton) {
     mobileStorageSettingsButton.addEventListener('click', function() {
       closeMobileMenu();
@@ -24045,6 +24097,179 @@ ${selector} .arrowheadPath {
     });
   }
 
+  // Visual Library Settings (Issue #129)
+  function populateVisualSettingsOptions() {
+    const options = (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getOptions === 'function')
+      ? window.visualSettingsAdapters.getOptions()
+      : (window.visualSettingsStore ? window.visualSettingsStore.DEFAULT_OPTIONS : null);
+
+    if (!options) return;
+
+    const list = [
+      { el: selectHljsTheme, opts: options.hljsTheme },
+      { el: selectMathFont, opts: options.mathFont },
+      { el: selectMermaidTheme, opts: options.mermaidTheme },
+      { el: selectMermaidLook, opts: options.mermaidLook },
+      { el: selectEmojiSkinTone, opts: options.emojiSkinTone }
+    ];
+
+    list.forEach(function(item) {
+      if (!item.el || !item.opts) return;
+      const currentVal = item.el.value;
+      item.el.innerHTML = '';
+      item.opts.forEach(function(opt) {
+        const val = typeof opt === 'object' && opt !== null ? opt.value : opt;
+        const label = typeof opt === 'object' && opt !== null ? opt.label : opt;
+        const optEl = document.createElement('option');
+        optEl.value = val;
+        optEl.textContent = label;
+        item.el.appendChild(optEl);
+      });
+      if (currentVal) item.el.value = currentVal;
+    });
+  }
+
+  function syncVisualSettingsUI() {
+    if (!window.visualSettingsStore) return;
+    const settings = window.visualSettingsStore.getSettings();
+    if (selectHljsTheme && settings.hljsTheme) selectHljsTheme.value = settings.hljsTheme;
+    if (selectMathFont && settings.mathFont) selectMathFont.value = settings.mathFont;
+    if (selectMermaidTheme && settings.mermaidTheme) selectMermaidTheme.value = settings.mermaidTheme;
+    if (selectMermaidLook && settings.mermaidLook) selectMermaidLook.value = settings.mermaidLook;
+    if (selectEmojiSkinTone && settings.emojiSkinTone) selectEmojiSkinTone.value = settings.emojiSkinTone;
+  }
+
+  function openVisualSettingsModal(opener) {
+    if (!visualSettingsModal) return;
+    populateVisualSettingsOptions();
+    syncVisualSettingsUI();
+    openAppModal(visualSettingsModal, {
+      focusTarget: selectHljsTheme,
+      returnFocus: opener || document.activeElement,
+      onClose: closeVisualSettingsModal
+    });
+  }
+
+  function closeVisualSettingsModal() {
+    if (visualSettingsModal) closeAppModal(visualSettingsModal);
+  }
+
+  let _visualSettingsDebounceTimer = null;
+
+  async function applyVisualSettings(settings, changedKey) {
+    if (window.visualSettingsAdapters) {
+      try {
+        if (changedKey === 'hljsTheme') {
+          await window.visualSettingsAdapters.applyHighlightTheme(settings.hljsTheme);
+        } else if (changedKey === 'mathFont') {
+          await window.visualSettingsAdapters.applyMathFont(settings.mathFont);
+        } else if (changedKey === 'mermaidTheme' || changedKey === 'mermaidLook') {
+          await window.visualSettingsAdapters.applyMermaid({
+            theme: settings.mermaidTheme,
+            look: settings.mermaidLook
+          });
+        } else if (changedKey === 'emojiSkinTone') {
+          await window.visualSettingsAdapters.applyEmojiSkinTone(settings.emojiSkinTone);
+        } else {
+          if (typeof window.visualSettingsAdapters.applyAll === 'function') {
+            await window.visualSettingsAdapters.applyAll(settings);
+          } else {
+            await Promise.all([
+              window.visualSettingsAdapters.applyHighlightTheme && window.visualSettingsAdapters.applyHighlightTheme(settings.hljsTheme),
+              window.visualSettingsAdapters.applyMathFont && window.visualSettingsAdapters.applyMathFont(settings.mathFont),
+              window.visualSettingsAdapters.applyMermaid && window.visualSettingsAdapters.applyMermaid({ theme: settings.mermaidTheme, look: settings.mermaidLook }),
+              window.visualSettingsAdapters.applyEmojiSkinTone && window.visualSettingsAdapters.applyEmojiSkinTone(settings.emojiSkinTone)
+            ]);
+          }
+        }
+      } catch (err) {
+        console.warn('Error applying visual setting via adapter:', err);
+      }
+    }
+
+    // Invalidate secondary split document preview cache (P6)
+    documentSplitPreviewContent = null;
+    if (secondarySplitTabId && typeof renderDocumentSplitView === 'function') {
+      renderDocumentSplitView();
+    }
+
+    // Check if re-render is needed according to adapter contract (e.g. hljsTheme does not need markdown AST re-render)
+    const needsRender = !window.visualSettingsAdapters ||
+      typeof window.visualSettingsAdapters.needsRerender !== 'function' ||
+      window.visualSettingsAdapters.needsRerender(changedKey);
+
+    if (needsRender) {
+      if (_visualSettingsDebounceTimer) {
+        clearTimeout(_visualSettingsDebounceTimer);
+      }
+      _visualSettingsDebounceTimer = setTimeout(function() {
+        _visualSettingsDebounceTimer = null;
+        renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+      }, 50);
+    }
+  }
+
+  if (visualSettingsButton) {
+    visualSettingsButton.addEventListener('click', function() {
+      openVisualSettingsModal(visualSettingsButton);
+    });
+  }
+
+  if (menuVisualSettingsButton) {
+    menuVisualSettingsButton.addEventListener('click', function() {
+      const settingsToggle = document.getElementById('workspaceSettingsDropdown');
+      if (settingsToggle && window.bootstrap && bootstrap.Dropdown) {
+        bootstrap.Dropdown.getOrCreateInstance(settingsToggle).hide();
+      }
+      openVisualSettingsModal(menuVisualSettingsButton);
+    });
+  }
+
+  if (visualSettingsCloseBtn) visualSettingsCloseBtn.addEventListener('click', closeVisualSettingsModal);
+  if (visualSettingsCloseIcon) visualSettingsCloseIcon.addEventListener('click', closeVisualSettingsModal);
+
+  if (selectHljsTheme) {
+    selectHljsTheme.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('hljsTheme', selectHljsTheme.value);
+    });
+  }
+  if (selectMathFont) {
+    selectMathFont.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mathFont', selectMathFont.value);
+    });
+  }
+  if (selectMermaidTheme) {
+    selectMermaidTheme.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mermaidTheme', selectMermaidTheme.value);
+    });
+  }
+  if (selectMermaidLook) {
+    selectMermaidLook.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mermaidLook', selectMermaidLook.value);
+    });
+  }
+  if (selectEmojiSkinTone) {
+    selectEmojiSkinTone.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('emojiSkinTone', selectEmojiSkinTone.value);
+    });
+  }
+
+  if (visualSettingsResetBtn) {
+    visualSettingsResetBtn.addEventListener('click', function() {
+      if (window.visualSettingsStore) {
+        window.visualSettingsStore.resetSettings();
+        syncVisualSettingsUI();
+      }
+    });
+  }
+
+  if (window.visualSettingsStore) {
+    window.visualSettingsStore.onSettingsChange(function(settings, changedKey) {
+      syncVisualSettingsUI();
+      applyVisualSettings(settings, changedKey);
+    });
+  }
+
   if (storageSettingsButton) {
     storageSettingsButton.addEventListener('click', function() {
       const settingsToggle = document.getElementById('workspaceSettingsDropdown');
@@ -24525,8 +24750,19 @@ ${selector} .arrowheadPath {
       }
     }
 
+    if (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.onAppThemeChange === 'function') {
+      window.visualSettingsAdapters.onAppThemeChange(theme);
+    }
+
     updateMapThemes();
     updateStlThemes();
+    if (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.onAppThemeChange === 'function') {
+      try {
+        window.visualSettingsAdapters.onAppThemeChange(theme);
+      } catch (err) {
+        console.warn('Error in visualSettingsAdapters.onAppThemeChange:', err);
+      }
+    }
   });
 
   async function nativeSaveMarkdown() {
@@ -32303,6 +32539,16 @@ ${selector} .arrowheadPath {
       activate: releaseNotesLaunch.activate
     });
   }
+
+  // Initialize Visual Library Settings (Issue #129)
+  window.renderMarkdown = renderMarkdown;
+  populateVisualSettingsOptions();
+  syncVisualSettingsUI();
+  if (window.visualSettingsStore) {
+    const initialVisualSettings = window.visualSettingsStore.getSettings();
+    await applyVisualSettings(initialVisualSettings, '*');
+  }
+
   document.documentElement.dataset.appReady = 'true';
   window.dispatchEvent(new Event('markdown-viewer:ready'));
 });
diff --git a/desktop-app/resources/js/visual-settings-adapters.js b/desktop-app/resources/js/visual-settings-adapters.js
new file mode 100644
index 00000000..bf05604f
--- /dev/null
+++ b/desktop-app/resources/js/visual-settings-adapters.js
@@ -0,0 +1,851 @@
+/**
+ * visual-settings-adapters.js
+ * Library adapters for Markdown Viewer visual settings (Issue #129).
+ * Manages runtime theming and visual configurations for:
+ * - highlight.js: theme stylesheets (auto, github, monokai, dracula, etc.)
+ * - MathJax: mathematical typesetting fonts (Modern, STIX2, Asana, Bonum, etc.)
+ * - Mermaid: diagram themes (auto, default, dark, forest, neutral, base) & looks (classic, handDrawn)
+ * - JoyPixels: emoji Fitzpatrick skin tones (default, tone1 - tone5)
+ */
+
+(function () {
+  'use strict';
+
+  // Allowed options metadata
+  const OPTIONS = Object.freeze({
+    hljsTheme: [
+      { value: 'auto', label: 'Auto (Follow App)' },
+      { value: 'github', label: 'GitHub' },
+      { value: 'github-dark', label: 'GitHub Dark' },
+      { value: 'monokai', label: 'Monokai' },
+      { value: 'dracula', label: 'Dracula' },
+      { value: 'atom-one-dark', label: 'Atom One Dark' },
+      { value: 'atom-one-light', label: 'Atom One Light' },
+      { value: 'solarized-dark', label: 'Solarized Dark' },
+      { value: 'solarized-light', label: 'Solarized Light' },
+      { value: 'vs', label: 'Visual Studio' },
+      { value: 'vs2015', label: 'Visual Studio 2015' },
+      { value: 'nord', label: 'Nord' },
+      { value: 'tokyo-night-dark', label: 'Tokyo Night Dark' },
+      { value: 'tokyo-night-light', label: 'Tokyo Night Light' },
+      { value: 'default', label: 'Default' }
+    ],
+    mathFont: [
+      { value: 'mathjax-modern', label: 'Computer Modern (TeX)' },
+      { value: 'mathjax-tex', label: 'TeX (Classic)' },
+      { value: 'mathjax-stix2', label: 'STIX Two' },
+      { value: 'mathjax-asana', label: 'Asana Math' },
+      { value: 'mathjax-bonum', label: 'Gyre Bonum' },
+      { value: 'mathjax-dejavu', label: 'DejaVu Math' },
+      { value: 'mathjax-fira', label: 'Fira Math' },
+      { value: 'mathjax-pagella', label: 'Gyre Pagella' },
+      { value: 'mathjax-schola', label: 'Gyre Schola' },
+      { value: 'mathjax-termes', label: 'Gyre Termes' }
+    ],
+    mermaidTheme: [
+      { value: 'auto', label: 'Auto (Follow App)' },
+      { value: 'default', label: 'Default' },
+      { value: 'neutral', label: 'Neutral' },
+      { value: 'dark', label: 'Dark' },
+      { value: 'forest', label: 'Forest' },
+      { value: 'base', label: 'Base' }
+    ],
+    mermaidLook: [
+      { value: 'classic', label: 'Classic' },
+      { value: 'handDrawn', label: 'Hand Drawn' }
+    ],
+    emojiSkinTone: [
+      { value: 'default', label: 'Default (Yellow)' },
+      { value: 'tone1', label: 'Light (Tone 1)' },
+      { value: 'tone2', label: 'Medium-Light (Tone 2)' },
+      { value: 'tone3', label: 'Medium (Tone 3)' },
+      { value: 'tone4', label: 'Medium-Dark (Tone 4)' },
+      { value: 'tone5', label: 'Dark (Tone 5)' }
+    ]
+  });
+
+  const DEFAULTS = Object.freeze({
+    hljsTheme: 'auto',
+    mathFont: 'mathjax-modern',
+    mermaidTheme: 'auto',
+    mermaidLook: 'classic',
+    emojiSkinTone: 'default'
+  });
+
+  // Active state
+  let _activeHljsTheme = DEFAULTS.hljsTheme;
+  let _activeMathFont = DEFAULTS.mathFont;
+  let _activeMermaidTheme = DEFAULTS.mermaidTheme;
+  let _activeMermaidLook = DEFAULTS.mermaidLook;
+  let _activeEmojiSkinTone = DEFAULTS.emojiSkinTone;
+
+  const HLJS_CDN_BASE = 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/';
+  const MATHJAX_FONT_CDN_BASE = 'https://cdn.jsdelivr.net/npm/';
+
+  // Mapping for highlight.js themes on cdnjs 11.9.0 (dracula & solarized live under base16/)
+  const HLJS_THEME_PATHS = Object.freeze({
+    'github': 'github.min.css',
+    'github-dark': 'github-dark.min.css',
+    'monokai': 'monokai.min.css',
+    'dracula': 'base16/dracula.min.css',
+    'atom-one-dark': 'atom-one-dark.min.css',
+    'atom-one-light': 'atom-one-light.min.css',
+    'solarized-dark': 'base16/solarized-dark.min.css',
+    'solarized-light': 'base16/solarized-light.min.css',
+    'vs': 'vs.min.css',
+    'vs2015': 'vs2015.min.css',
+    'nord': 'nord.min.css',
+    'tokyo-night-dark': 'tokyo-night-dark.min.css',
+    'tokyo-night-light': 'tokyo-night-light.min.css',
+    'default': 'default.min.css'
+  });
+
+  // Fallback background and foreground colors for syntax themes
+  const HLJS_THEME_COLORS = Object.freeze({
+    'github': { bg: '#ffffff', color: '#24292e' },
+    'github-dark': { bg: '#0d1117', color: '#c9d1d9' },
+    'monokai': { bg: '#272822', color: '#dddddd' },
+    'dracula': { bg: '#282a36', color: '#f8f8f2' },
+    'atom-one-dark': { bg: '#282c34', color: '#abb2bf' },
+    'atom-one-light': { bg: '#fafafa', color: '#383a42' },
+    'solarized-dark': { bg: '#002b36', color: '#839496' },
+    'solarized-light': { bg: '#fdf6e3', color: '#657b83' },
+    'vs': { bg: '#ffffff', color: '#000000' },
+    'vs2015': { bg: '#1e1e1e', color: '#dcdcdc' },
+    'nord': { bg: '#2e3440', color: '#d8dee9' },
+    'tokyo-night-dark': { bg: '#1a1b26', color: '#a9b1d6' },
+    'tokyo-night-light': { bg: '#d5d6db', color: '#343b58' },
+    'default': { bg: '#f3f3f3', color: '#444444' }
+  });
+
+  function getEffectiveAppTheme() {
+    if (typeof document === 'undefined' || !document.documentElement) return 'light';
+    return document.documentElement.getAttribute('data-theme') === 'dark' ? 'dark' : 'light';
+  }
+
+  function resolveHljsThemeName(theme, themeOverride) {
+    if (theme === 'auto') {
+      const effective = themeOverride || getEffectiveAppTheme();
+      return effective === 'dark' ? 'github-dark' : 'github';
+    }
+    return theme || 'github';
+  }
+
+  function resolveMermaidThemeName(theme, themeOverride) {
+    if (theme === 'auto') {
+      const effective = themeOverride || getEffectiveAppTheme();
+      return effective === 'dark' ? 'dark' : 'default';
+    }
+    return theme || 'default';
+  }
+
+  // Update dynamic helper stylesheet so code blocks adopt the theme's background and contrast
+  function updateHljsHelperStyle(effectiveName) {
+    if (typeof document === 'undefined') return;
+    let style = document.getElementById('mdv-hljs-style-helper');
+    if (!style) {
+      style = document.createElement('style');
+      style.id = 'mdv-hljs-style-helper';
+      if (document.head) document.head.appendChild(style);
+    }
+
+    const fallback = HLJS_THEME_COLORS[effectiveName] || HLJS_THEME_COLORS['github'];
+    let bg = fallback.bg;
+    let fg = fallback.color;
+
+    if (document.body && typeof window !== 'undefined' && typeof window.getComputedStyle === 'function') {
+      try {
+        const probe = document.createElement('div');
+        probe.className = 'hljs';
+        probe.style.position = 'fixed';
+        probe.style.top = '-9999px';
+        probe.style.left = '-9999px';
+        probe.style.visibility = 'hidden';
+        document.body.appendChild(probe);
+        const cs = window.getComputedStyle(probe);
+        if (cs && cs.backgroundColor && cs.backgroundColor !== 'rgba(0, 0, 0, 0)' && cs.backgroundColor !== 'transparent') {
+          bg = cs.backgroundColor;
+        }
+        if (cs && cs.color && cs.color !== 'rgba(0, 0, 0, 0)' && cs.color !== 'transparent') {
+          fg = cs.color;
+        }
+        document.body.removeChild(probe);
+      } catch (_) {}
+    }
+
+    if (_activeHljsTheme === 'auto') {
+      style.textContent = `
+        /* Preview code blocks in Auto mode adopt application theme */
+        .markdown-body:not(.pdf-export):not(.export-render-root) pre:has(> code.hljs) {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .markdown-body:not(.pdf-export):not(.export-render-root) pre > code.hljs,
+        .markdown-body:not(.pdf-export):not(.export-render-root) code.hljs {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        /* Export documents in Auto mode resolve against the selected export theme */
+        .pdf-export.theme-dark pre:has(> code.hljs),
+        .pdf-export[data-theme="dark"] pre:has(> code.hljs),
+        .export-render-root[data-theme="dark"] pre:has(> code.hljs),
+        .export-render-root.theme-dark pre:has(> code.hljs) {
+          background-color: #161b22 !important;
+          color: #c9d1d9 !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .pdf-export.theme-dark pre > code.hljs,
+        .pdf-export[data-theme="dark"] pre > code.hljs,
+        .export-render-root[data-theme="dark"] pre > code.hljs,
+        .export-render-root.theme-dark pre > code.hljs,
+        .pdf-export.theme-dark code.hljs,
+        .pdf-export[data-theme="dark"] code.hljs,
+        .export-render-root[data-theme="dark"] code.hljs,
+        .export-render-root.theme-dark code.hljs {
+          background-color: #161b22 !important;
+          color: #c9d1d9 !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        .pdf-export.theme-light pre:has(> code.hljs),
+        .pdf-export[data-theme="light"] pre:has(> code.hljs),
+        .export-render-root[data-theme="light"] pre:has(> code.hljs),
+        .export-render-root.theme-light pre:has(> code.hljs) {
+          background-color: #f6f8fa !important;
+          color: #24292e !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .pdf-export.theme-light pre > code.hljs,
+        .pdf-export[data-theme="light"] pre > code.hljs,
+        .export-render-root[data-theme="light"] pre > code.hljs,
+        .export-render-root.theme-light pre > code.hljs,
+        .pdf-export.theme-light code.hljs,
+        .pdf-export[data-theme="light"] code.hljs,
+        .export-render-root[data-theme="light"] code.hljs,
+        .export-render-root.theme-light code.hljs {
+          background-color: #f6f8fa !important;
+          color: #24292e !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        @media print {
+          .markdown-body:not(.pdf-export):not(.export-render-root) pre:has(> code.hljs),
+          .markdown-body:not(.pdf-export):not(.export-render-root) pre > code.hljs,
+          .markdown-body:not(.pdf-export):not(.export-render-root) code.hljs {
+            background-color: ${bg} !important;
+            color: ${fg} !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+
+          html[data-browser-print-export="dark"] .markdown-body pre:has(> code.hljs),
+          html[data-browser-print-export="dark"] .markdown-body pre > code.hljs,
+          html[data-browser-print-export="dark"] .markdown-body code.hljs,
+          .pdf-export.theme-dark pre:has(> code.hljs),
+          .pdf-export.theme-dark pre > code.hljs,
+          .pdf-export.theme-dark code.hljs {
+            background-color: #161b22 !important;
+            color: #c9d1d9 !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light pre:has(> code.hljs),
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light pre > code.hljs,
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light code.hljs,
+          html[data-browser-print-export="light"] .markdown-body pre:has(> code.hljs),
+          html[data-browser-print-export="light"] .markdown-body pre > code.hljs,
+          html[data-browser-print-export="light"] .markdown-body code.hljs,
+          .pdf-export.theme-light pre:has(> code.hljs),
+          .pdf-export.theme-light pre > code.hljs,
+          .pdf-export.theme-light code.hljs {
+            background-color: #f6f8fa !important;
+            color: #24292e !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+        }
+      `;
+    } else {
+      style.textContent = `
+        .markdown-body pre:has(> code.hljs) {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .markdown-body pre > code.hljs,
+        .markdown-body code.hljs {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+        @media print {
+          .markdown-body pre:has(> code.hljs),
+          .markdown-body pre > code.hljs,
+          .markdown-body code.hljs {
+            background-color: ${bg} !important;
+            color: ${fg} !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+        }
+      `;
+    }
+  }
+
+  /**
+   * Apply highlight.js theme stylesheet.
+   * Dynamically manages the <link id="mdv-hljs-theme-link"> in <head>.
+   */
+  function applyHighlightTheme(theme) {
+    _activeHljsTheme = theme || DEFAULTS.hljsTheme;
+    const effectiveName = resolveHljsThemeName(_activeHljsTheme);
+    updateHljsHelperStyle(effectiveName);
+
+    if (typeof document === 'undefined') return Promise.resolve();
+
+    if (document.documentElement) {
+      if (_activeHljsTheme !== 'auto') {
+        document.documentElement.setAttribute('data-custom-syntax-theme', _activeHljsTheme);
+        document.documentElement.setAttribute('data-hljs-theme', _activeHljsTheme);
+      } else {
+        if (typeof document.documentElement.removeAttribute === 'function') {
+          document.documentElement.removeAttribute('data-custom-syntax-theme');
+        }
+        document.documentElement.setAttribute('data-hljs-theme', 'auto');
+      }
+    }
+
+    const subpath = HLJS_THEME_PATHS[effectiveName] || `${effectiveName}.min.css`;
+    const href = `${HLJS_CDN_BASE}${subpath}`;
+
+    let link = document.getElementById('mdv-hljs-theme-link');
+    if (!link) {
+      link = document.createElement('link');
+      link.id = 'mdv-hljs-theme-link';
+      link.rel = 'stylesheet';
+      link.crossOrigin = 'anonymous';
+      if (document.head) document.head.appendChild(link);
+    }
+
+    if (link.getAttribute('href') === href) {
+      updateHljsHelperStyle(effectiveName);
+      return Promise.resolve();
+    }
+
+    return new Promise((resolve) => {
+      link.onload = () => {
+        updateHljsHelperStyle(effectiveName);
+        resolve();
+      };
+      link.onerror = () => {
+        console.warn(`[VisualAdapters] Failed to load highlight.js theme: ${href}`);
+        updateHljsHelperStyle(effectiveName);
+        resolve(); // resolve gracefully so UI does not stall
+      };
+      link.href = href;
+    });
+  }
+
+  /**
+   * Dynamically loads a script tag if not already loaded.
+   */
+  const _loadedScripts = new Set();
+  function loadScriptOnce(src) {
+    if (_loadedScripts.has(src)) return Promise.resolve();
+    if (typeof document === 'undefined') return Promise.resolve();
+    return new Promise((resolve, reject) => {
+      const existing = document.querySelector(`script[src="${src}"]`);
+      if (existing) {
+        _loadedScripts.add(src);
+        return resolve();
+      }
+      const script = document.createElement('script');
+      script.src = src;
+      script.charset = 'UTF-8';
+      script.crossOrigin = 'anonymous';
+      script.onload = () => {
+        _loadedScripts.add(src);
+        resolve();
+      };
+      script.onerror = (err) => {
+        console.warn(`[VisualAdapters] Script load failed: ${src}`, err);
+        reject(err);
+      };
+      if (document.head) document.head.appendChild(script);
+      else if (document.body) document.body.appendChild(script);
+    });
+  }
+
+  /**
+   * Apply MathJax mathematical font.
+   */
+  async function applyMathFont(font) {
+    _activeMathFont = font || DEFAULTS.mathFont;
+
+    if (typeof window === 'undefined' || !window.MathJax) {
+      return;
+    }
+
+    const isMathJax3 = (window.MathJax.version && String(window.MathJax.version).startsWith('3.')) ||
+                       (window.MathJax._ && !window.MathJax._.output?.fonts);
+
+    // Configure font and complete resource configuration in MathJax
+    const fontPkgUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font`;
+    const fontWoffUrl = `${fontPkgUrl}/chtml/woff`;
+    const fontDynamicPrefix = `[${_activeMathFont}]/chtml/dynamic`;
+
+    window.MathJax.output = window.MathJax.output || {};
+    window.MathJax.output.font = _activeMathFont;
+
+    window.MathJax.config = window.MathJax.config || {};
+    window.MathJax.config.output = window.MathJax.config.output || {};
+    window.MathJax.config.output.font = _activeMathFont;
+
+    window.MathJax.chtml = window.MathJax.chtml || {};
+    window.MathJax.config.chtml = window.MathJax.config.chtml || {};
+    if (typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
+    if (typeof window.MathJax.config.chtml.font === 'string') {
+      delete window.MathJax.config.chtml.font;
+    }
+
+    if (!isMathJax3) {
+      window.MathJax.chtml.fontURL = fontWoffUrl;
+      window.MathJax.chtml.dynamicPrefix = fontDynamicPrefix;
+      window.MathJax.config.chtml.fontURL = fontWoffUrl;
+      window.MathJax.config.chtml.dynamicPrefix = fontDynamicPrefix;
+
+      window.MathJax.loader = window.MathJax.loader || {};
+      window.MathJax.loader.paths = window.MathJax.loader.paths || {};
+      window.MathJax.loader.paths[_activeMathFont] = fontPkgUrl;
+
+      window.MathJax.config.loader = window.MathJax.config.loader || {};
+      window.MathJax.config.loader.paths = window.MathJax.config.loader.paths || {};
+      window.MathJax.config.loader.paths[_activeMathFont] = fontPkgUrl;
+
+      if (window.MathJax._ && window.MathJax._.components && window.MathJax._.components.loader && window.MathJax._.components.loader.CONFIG && window.MathJax._.components.loader.CONFIG.paths) {
+        window.MathJax._.components.loader.CONFIG.paths[_activeMathFont] = fontPkgUrl;
+      }
+    }
+
+    // Load external font script if needed and MathJax core is already loaded.
+    // If MathJax has not loaded yet, configuring output.font and resource URLs
+    // is sufficient because MathJax's startup loader will load the font component itself.
+    const fontAlreadyLoaded = !!(
+      window.MathJax._ &&
+      window.MathJax._.output &&
+      window.MathJax._.output.fonts &&
+      window.MathJax._.output.fonts[_activeMathFont]
+    );
+
+    if (!fontAlreadyLoaded && window.MathJax._ && !isMathJax3) {
+      const fontUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font/chtml.js`;
+      try {
+        await loadScriptOnce(fontUrl);
+      } catch (err) {
+        console.warn(`[VisualAdapters] MathJax font load failed (${_activeMathFont}), falling back to bundled Modern:`, err);
+        _activeMathFont = 'mathjax-modern';
+        if (
+          window.MathJax._ &&
+          (!window.MathJax._.output || !window.MathJax._.output.fonts || !window.MathJax._.output.fonts['mathjax-modern'])
+        ) {
+          try {
+            await loadScriptOnce(`${MATHJAX_FONT_CDN_BASE}mathjax-modern-font/chtml.js`);
+          } catch (modernErr) {
+            console.warn('[VisualAdapters] MathJax modern fallback load failed:', modernErr);
+          }
+        }
+      }
+    }
+
+    // Extract registered font class and activate its fontData
+    let fontClass = null;
+    if (window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts) {
+      const fontPkg = window.MathJax._.output.fonts[_activeMathFont];
+      if (fontPkg && fontPkg.chtml_ts) {
+        fontClass = Object.values(fontPkg.chtml_ts)[0] || null;
+      }
+    }
+
+    // If font class was not found (e.g. offline fallback), fall back to bundled modern font
+    if (!fontClass && _activeMathFont !== 'mathjax-modern' && window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts && window.MathJax._.output.fonts['mathjax-modern']) {
+      _activeMathFont = 'mathjax-modern';
+      const modernPkg = window.MathJax._.output.fonts['mathjax-modern'];
+      if (modernPkg.chtml_ts) {
+        fontClass = Object.values(modernPkg.chtml_ts)[0] || null;
+      }
+    }
+
+    // Update MathJax configuration with effective font and complete resource configuration
+    const effectivePkgUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font`;
+    const effectiveWoffUrl = `${effectivePkgUrl}/chtml/woff`;
+    const effectiveDynamicPrefix = `[${_activeMathFont}]/chtml/dynamic`;
+
+    window.MathJax.config.output.font = _activeMathFont;
+    if (typeof window.MathJax.config.chtml.font === 'string') {
+      delete window.MathJax.config.chtml.font;
+    }
+    if (typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
+    window.MathJax.config.chtml.fontURL = effectiveWoffUrl;
+    window.MathJax.config.chtml.dynamicPrefix = effectiveDynamicPrefix;
+    window.MathJax.chtml.fontURL = effectiveWoffUrl;
+    window.MathJax.chtml.dynamicPrefix = effectiveDynamicPrefix;
+
+    window.MathJax.config.loader = window.MathJax.config.loader || {};
+    window.MathJax.config.loader.paths = window.MathJax.config.loader.paths || {};
+    window.MathJax.config.loader.paths[_activeMathFont] = effectivePkgUrl;
+    window.MathJax.loader = window.MathJax.loader || {};
+    window.MathJax.loader.paths = window.MathJax.loader.paths || {};
+    window.MathJax.loader.paths[_activeMathFont] = effectivePkgUrl;
+    if (window.MathJax._ && window.MathJax._.components && window.MathJax._.components.loader && window.MathJax._.components.loader.CONFIG && window.MathJax._.components.loader.CONFIG.paths) {
+      window.MathJax._.components.loader.CONFIG.paths[_activeMathFont] = effectivePkgUrl;
+    }
+
+    if (fontClass) {
+      window.MathJax.config.chtml.fontData = fontClass;
+      window.MathJax.chtml.fontData = fontClass;
+    } else if (_activeMathFont === 'mathjax-modern' && window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts && window.MathJax._.output.fonts['mathjax-modern']) {
+      const modernPkg = window.MathJax._.output.fonts['mathjax-modern'];
+      if (modernPkg.chtml_ts) {
+        window.MathJax.config.chtml.fontData = Object.values(modernPkg.chtml_ts)[0] || null;
+        window.MathJax.chtml.fontData = window.MathJax.config.chtml.fontData;
+      }
+    }
+
+    // If MathJax startup has completed, switch output jax, document and re-typeset
+    if (!isMathJax3 && window.MathJax.startup && typeof window.MathJax.startup.getOutputJax === 'function') {
+      try {
+        const newOutputJax = window.MathJax.startup.getOutputJax();
+        if (newOutputJax) {
+          if (newOutputJax.font && newOutputJax.font.options) {
+            newOutputJax.font.options.fontURL = effectiveWoffUrl;
+            newOutputJax.font.options.dynamicPrefix = effectiveDynamicPrefix;
+          }
+          window.MathJax.startup.output = newOutputJax;
+          if (typeof window.MathJax.startup.getDocument === 'function') {
+            window.MathJax.startup.document = window.MathJax.startup.getDocument();
+          }
+        }
+
+        // Re-render math in preview if present
+        if (typeof document !== 'undefined') {
+          const preview = document.getElementById('markdown-preview');
+          const splitPreview = document.getElementById('document-split-preview');
+          const roots = [];
+          if (preview && preview.querySelector('mjx-container, .math-block, .math-inline')) roots.push(preview);
+          if (splitPreview && splitPreview.querySelector('mjx-container, .math-block, .math-inline')) roots.push(splitPreview);
+
+          if (roots.length > 0) {
+            if (typeof window.renderMarkdown === 'function') {
+              window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+            } else if (typeof window.MathJax.typesetPromise === 'function') {
+              await window.MathJax.typesetPromise(roots);
+            }
+          }
+        }
+      } catch (err) {
+        console.warn('[VisualAdapters] MathJax font switch error:', err);
+      }
+    }
+  }
+
+  /**
+   * Apply Mermaid theme and look drawing style.
+   */
+  async function applyMermaid(options) {
+    if (options) {
+      if (typeof options.theme !== 'undefined') _activeMermaidTheme = options.theme;
+      if (typeof options.look !== 'undefined') _activeMermaidLook = options.look;
+    }
+
+    if (typeof mermaid === 'undefined') {
+      return;
+    }
+
+    const effectiveTheme = resolveMermaidThemeName(_activeMermaidTheme);
+    const effectiveLook = _activeMermaidLook === 'handDrawn' ? 'handDrawn' : 'classic';
+
+    try {
+      mermaid.initialize({
+        startOnLoad: false,
+        theme: effectiveTheme,
+        look: effectiveLook,
+        securityLevel: 'strict',
+        flowchart: { useMaxWidth: true, htmlLabels: true },
+        fontSize: 16,
+        gantt: { useWidth: 1200 }
+      });
+    } catch (err) {
+      console.warn('[VisualAdapters] Mermaid initialize error:', err);
+    }
+
+    // Re-render existing Mermaid nodes in the preview
+    if (typeof document !== 'undefined') {
+      const preview = document.getElementById('markdown-preview');
+      if (preview) {
+        const mermaidNodes = preview.querySelectorAll('.mermaid');
+        if (mermaidNodes.length > 0) {
+          mermaidNodes.forEach((node) => {
+            const originalCode = node.getAttribute('data-original-code');
+            if (originalCode) {
+              node.innerHTML = decodeURIComponent(originalCode)
+                .replace(/&/g, '&amp;')
+                .replace(/</g, '&lt;')
+                .replace(/>/g, '&gt;');
+            }
+            node.removeAttribute('data-processed');
+
+            const container = node.closest('.mermaid-container');
+            if (container) {
+              const oldToolbar = container.querySelector('.mermaid-toolbar');
+              if (oldToolbar) oldToolbar.remove();
+            }
+          });
+
+          if (typeof window.renderMarkdown === 'function') {
+            window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+          }
+        }
+      }
+    }
+  }
+
+  /**
+   * Resolves a shortcode to JoyPixels' canonical shortcode name (e.g. '+1' -> 'thumbsup', 'runner' -> 'person_running').
+   */
+  function getCanonicalEmojiShortcode(code, joy) {
+    if (!code) return code;
+    const clean = code.replace(/^:|:$/g, '');
+    if (!joy) return clean;
+    const colonCode = `:${clean}:`;
+
+    // 1. If it's already a canonical key in joy.emojiList
+    if (joy.emojiList && joy.emojiList[colonCode]) {
+      return clean;
+    }
+
+    // 2. Check joy.altShortNames (native JoyPixels alias map)
+    if (joy.altShortNames) {
+      const alt = joy.altShortNames[colonCode] || joy.altShortNames[clean];
+      if (alt) {
+        return alt.replace(/^:|:$/g, '');
+      }
+    }
+
+    // 3. Fallback: scan emojiList if altShortNames is not populated
+    if (joy.emojiList) {
+      for (const [key, val] of Object.entries(joy.emojiList)) {
+        if (val && Array.isArray(val.shortnames)) {
+          if (val.shortnames.includes(colonCode) || val.shortnames.includes(clean)) {
+            return key.replace(/^:|:$/g, '');
+          }
+        }
+      }
+    }
+
+    return clean;
+  }
+
+  /**
+   * Resolves an emoji shortcode to include the active Fitzpatrick skin tone modifier
+   * if the emoji supports skin tone modifiers, otherwise returns the original shortcode.
+   * Canonicalizes JoyPixels aliases prior to checking tone support.
+   * Shortcodes with explicit tone modifiers are left untouched.
+   */
+  function resolveEmojiShortcode(shortcode) {
+    if (!shortcode) return shortcode;
+    const clean = shortcode.replace(/^:|:$/g, '');
+
+    if (!_activeEmojiSkinTone || _activeEmojiSkinTone === 'default') {
+      return shortcode;
+    }
+
+    // If already has an explicit skin tone suffix, preserve it
+    if (/_tone[1-5]$/.test(clean)) {
+      return shortcode;
+    }
+
+    const joy = typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined);
+    const canonical = getCanonicalEmojiShortcode(clean, joy);
+    const toneCandidate = `${canonical}_${_activeEmojiSkinTone}`;
+
+    // Check JoyPixels library list if available
+    if (joy && joy.emojiList) {
+      if (joy.emojiList[`:${toneCandidate}:`]) {
+        return toneCandidate;
+      }
+      const directCandidate = `${clean}_${_activeEmojiSkinTone}`;
+      if (joy.emojiList[`:${directCandidate}:`]) {
+        return directCandidate;
+      }
+      return shortcode;
+    }
+
+    return toneCandidate;
+  }
+
+  /**
+   * Apply JoyPixels emoji skin tone.
+   */
+  function applyEmojiSkinTone(tone) {
+    _activeEmojiSkinTone = tone || DEFAULTS.emojiSkinTone;
+
+    const joy = typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined);
+    if (joy && typeof joy.shortnameToUnicode === 'function' && !joy._visualAdapterWrapped) {
+      const origShortnameToUnicode = joy.shortnameToUnicode.bind(joy);
+      joy.shortnameToUnicode = function (str) {
+        if (_activeEmojiSkinTone && _activeEmojiSkinTone !== 'default') {
+          str = str.replace(/:([\w+-]+):/g, function (match, code) {
+            const resolved = resolveEmojiShortcode(code);
+            return `:${resolved}:`;
+          });
+        }
+        return origShortnameToUnicode(str);
+      };
+      joy._visualAdapterWrapped = true;
+    }
+
+    if (typeof document !== 'undefined') {
+      const preview = document.getElementById('markdown-preview');
+      if (preview && typeof window.renderMarkdown === 'function') {
+        window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+      }
+    }
+  }
+
+  /**
+   * React to application light/dark theme toggle.
+   * Idempotently re-applies any setting configured as 'auto'.
+   */
+  function onAppThemeChange(newAppTheme) {
+    if (_activeHljsTheme === 'auto') {
+      applyHighlightTheme('auto');
+    }
+    if (_activeMermaidTheme === 'auto') {
+      applyMermaid({ theme: 'auto', look: _activeMermaidLook });
+    }
+  }
+
+  // Observe theme changes on documentElement
+  if (typeof MutationObserver !== 'undefined' && typeof document !== 'undefined' && document.documentElement) {
+    const themeObserver = new MutationObserver((mutations) => {
+      for (const mutation of mutations) {
+        if (mutation.type === 'attributes' && mutation.attributeName === 'data-theme') {
+          onAppThemeChange(getEffectiveAppTheme());
+        }
+      }
+    });
+
+    themeObserver.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ['data-theme']
+    });
+  }
+
+  /**
+   * Check if setting change needs a markdown document re-render.
+   */
+  function needsRerender(key) {
+    if (key === 'hljsTheme') return false;
+    return true;
+  }
+
+  /**
+   * Generic apply method for any setting key.
+   */
+  async function apply(key, value) {
+    if (key === 'hljsTheme') {
+      return applyHighlightTheme(value);
+    } else if (key === 'mathFont') {
+      return applyMathFont(value);
+    } else if (key === 'mermaidTheme') {
+      return applyMermaid({ theme: value, look: _activeMermaidLook });
+    } else if (key === 'mermaidLook') {
+      return applyMermaid({ theme: _activeMermaidTheme, look: value });
+    } else if (key === 'emojiSkinTone') {
+      return applyEmojiSkinTone(value);
+    }
+  }
+
+  /**
+   * Apply all settings simultaneously.
+   */
+  async function applyAll(settings) {
+    if (!settings) return;
+    const tasks = [];
+    if (typeof settings.hljsTheme !== 'undefined') {
+      tasks.push(applyHighlightTheme(settings.hljsTheme));
+    }
+    if (typeof settings.mathFont !== 'undefined') {
+      tasks.push(applyMathFont(settings.mathFont));
+    }
+    if (typeof settings.mermaidTheme !== 'undefined' || typeof settings.mermaidLook !== 'undefined') {
+      tasks.push(applyMermaid({
+        theme: settings.mermaidTheme !== undefined ? settings.mermaidTheme : _activeMermaidTheme,
+        look: settings.mermaidLook !== undefined ? settings.mermaidLook : _activeMermaidLook
+      }));
+    }
+    if (typeof settings.emojiSkinTone !== 'undefined') {
+      tasks.push(applyEmojiSkinTone(settings.emojiSkinTone));
+    }
+
+    await Promise.all(tasks);
+  }
+
+  // Public API exported to window.visualSettingsAdapters
+  const adapters = {
+    getOptions: () => JSON.parse(JSON.stringify(OPTIONS)),
+    getDefaults: () => Object.assign({}, DEFAULTS),
+    getCurrentSettings: () => ({
+      hljsTheme: _activeHljsTheme,
+      mathFont: _activeMathFont,
+      mermaidTheme: _activeMermaidTheme,
+      mermaidLook: _activeMermaidLook,
+      emojiSkinTone: _activeEmojiSkinTone
+    }),
+    getCurrentMathFont: () => _activeMathFont,
+    getMermaidConfig: (themeOverride) => ({
+      theme: resolveMermaidThemeName(_activeMermaidTheme, themeOverride),
+      look: _activeMermaidLook === 'handDrawn' ? 'handDrawn' : 'classic'
+    }),
+    getHljsTheme: (themeOverride) => resolveHljsThemeName(_activeHljsTheme, themeOverride),
+    resolveHljsThemeName,
+    needsRerender,
+    apply,
+    getCanonicalEmojiShortcode: (code) => getCanonicalEmojiShortcode(code, typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined)),
+    resolveEmojiShortcode,
+    applyHighlightTheme,
+    applyMathFont,
+    applyMermaid,
+    applyEmojiSkinTone,
+    applyAll,
+    onAppThemeChange
+  };
+
+  if (typeof window !== 'undefined') {
+    window.visualSettingsAdapters = adapters;
+  }
+  if (typeof module !== 'undefined' && module.exports) {
+    module.exports = adapters;
+  }
+
+})();
diff --git a/desktop-app/resources/js/visual-settings-store.js b/desktop-app/resources/js/visual-settings-store.js
new file mode 100644
index 00000000..a73167be
--- /dev/null
+++ b/desktop-app/resources/js/visual-settings-store.js
@@ -0,0 +1,183 @@
+(function () {
+  'use strict';
+
+  const STORAGE_KEY = 'mdv.visualSettings.v1';
+
+  const DEFAULT_SETTINGS = Object.freeze({
+    hljsTheme: 'auto',
+    mathFont: 'mathjax-modern',
+    mermaidTheme: 'auto',
+    mermaidLook: 'classic',
+    emojiSkinTone: 'default'
+  });
+
+  const DEFAULT_OPTIONS = Object.freeze({
+    hljsTheme: Object.freeze([
+      'auto', 'github', 'github-dark', 'monokai', 'dracula',
+      'atom-one-dark', 'atom-one-light', 'solarized-dark', 'solarized-light',
+      'vs', 'vs2015', 'nord', 'tokyo-night-dark', 'tokyo-night-light', 'default'
+    ]),
+    mathFont: Object.freeze([
+      'mathjax-modern', 'mathjax-tex', 'mathjax-stix2', 'mathjax-asana',
+      'mathjax-bonum', 'mathjax-dejavu', 'mathjax-fira', 'mathjax-pagella',
+      'mathjax-schola', 'mathjax-termes'
+    ]),
+    mermaidTheme: Object.freeze([
+      'auto', 'default', 'neutral', 'dark', 'forest', 'base'
+    ]),
+    mermaidLook: Object.freeze([
+      'classic', 'handDrawn'
+    ]),
+    emojiSkinTone: Object.freeze([
+      'default', 'tone1', 'tone2', 'tone3', 'tone4', 'tone5'
+    ])
+  });
+
+  function getEffectiveDefaults() {
+    if (typeof window !== 'undefined' && window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getDefaults === 'function') {
+      try {
+        const defs = window.visualSettingsAdapters.getDefaults();
+        if (defs && typeof defs === 'object') {
+          return Object.assign({}, DEFAULT_SETTINGS, defs);
+        }
+      } catch (e) {
+        console.warn('Error reading defaults from visualSettingsAdapters:', e);
+      }
+    }
+    return Object.assign({}, DEFAULT_SETTINGS);
+  }
+
+  function getAllowedValues(key) {
+    if (typeof window !== 'undefined' && window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getOptions === 'function') {
+      try {
+        const opts = window.visualSettingsAdapters.getOptions();
+        if (opts && Array.isArray(opts[key])) {
+          return opts[key].map(item => (typeof item === 'object' && item !== null ? item.value : item));
+        }
+      } catch (e) {
+        console.warn('Error reading options from visualSettingsAdapters:', e);
+      }
+    }
+    return DEFAULT_OPTIONS[key] || [];
+  }
+
+  function validateSettings(settings) {
+    const defaults = getEffectiveDefaults();
+    const valid = {};
+    for (const key of Object.keys(DEFAULT_SETTINGS)) {
+      const allowed = getAllowedValues(key);
+      const val = settings && settings[key];
+      if (typeof val === 'string' && allowed.includes(val)) {
+        valid[key] = val;
+      } else {
+        valid[key] = defaults[key];
+      }
+    }
+    return valid;
+  }
+
+  function loadFromStorage() {
+    const defaults = getEffectiveDefaults();
+    try {
+      if (typeof localStorage === 'undefined') return defaults;
+      const raw = localStorage.getItem(STORAGE_KEY);
+      if (!raw) return defaults;
+      const parsed = JSON.parse(raw);
+      return validateSettings(parsed);
+    } catch (e) {
+      console.warn('Failed to parse visual settings from localStorage:', e);
+      return defaults;
+    }
+  }
+
+  function saveToStorage(settings) {
+    try {
+      if (typeof localStorage !== 'undefined') {
+        localStorage.setItem(STORAGE_KEY, JSON.stringify(settings));
+      }
+    } catch (e) {
+      console.warn('Failed to save visual settings to localStorage:', e);
+    }
+  }
+
+  let currentSettings = loadFromStorage();
+  const listeners = new Set();
+
+  function notify(changedKey, changedValue) {
+    const settingsSnapshot = getSettings();
+    listeners.forEach(cb => {
+      try {
+        cb(settingsSnapshot, changedKey, changedValue);
+      } catch (e) {
+        console.error('Error in visual settings change listener:', e);
+      }
+    });
+  }
+
+  function getSettings() {
+    return Object.assign({}, currentSettings);
+  }
+
+  function updateSetting(key, value) {
+    if (!Object.prototype.hasOwnProperty.call(DEFAULT_SETTINGS, key)) {
+      console.warn('Unknown visual setting key:', key);
+      return false;
+    }
+    const allowed = getAllowedValues(key);
+    if (!allowed.includes(value)) {
+      console.warn(`Invalid value "${value}" for visual setting "${key}"`);
+      return false;
+    }
+    if (currentSettings[key] === value) {
+      return true;
+    }
+    currentSettings[key] = value;
+    saveToStorage(currentSettings);
+    notify(key, value);
+    return true;
+  }
+
+  function resetSettings() {
+    const defaults = getEffectiveDefaults();
+    currentSettings = Object.assign({}, defaults);
+    saveToStorage(currentSettings);
+    notify('*', null);
+    return getSettings();
+  }
+
+  function onSettingsChange(cb) {
+    if (typeof cb === 'function') {
+      listeners.add(cb);
+    }
+    return function unsubscribe() {
+      listeners.delete(cb);
+    };
+  }
+
+  // Refresh settings when adapters become available or options change
+  function refreshValidation() {
+    currentSettings = validateSettings(currentSettings);
+    saveToStorage(currentSettings);
+  }
+
+  const store = {
+    STORAGE_KEY,
+    DEFAULT_SETTINGS,
+    DEFAULT_OPTIONS,
+    getSettings,
+    updateSetting,
+    resetSettings,
+    onSettingsChange,
+    validateSettings,
+    getAllowedValues,
+    refreshValidation
+  };
+
+  if (typeof window !== 'undefined') {
+    window.visualSettingsStore = store;
+  }
+
+  if (typeof module !== 'undefined' && module.exports) {
+    module.exports = store;
+  }
+})();
diff --git a/desktop-app/resources/styles.css b/desktop-app/resources/styles.css
index 5671d94c..afce7f22 100644
--- a/desktop-app/resources/styles.css
+++ b/desktop-app/resources/styles.css
@@ -4112,94 +4112,94 @@ a:focus {
   background-color: var(--code-bg);
 }
 
-/* Syntax Highlighting Mapping to GitHub Variables */
-.hljs {
+/* Syntax Highlighting Mapping to GitHub Variables (scoped to when no external syntax theme is selected) */
+html:not([data-custom-syntax-theme]) .hljs {
   color: var(--color-fg-default);
 }
 
-.hljs-doctag,
-.hljs-keyword,
-.hljs-meta .hljs-keyword,
-.hljs-template-tag,
-.hljs-template-variable,
-.hljs-type,
-.hljs-variable.language_ {
+html:not([data-custom-syntax-theme]) .hljs-doctag,
+html:not([data-custom-syntax-theme]) .hljs-keyword,
+html:not([data-custom-syntax-theme]) .hljs-meta .hljs-keyword,
+html:not([data-custom-syntax-theme]) .hljs-template-tag,
+html:not([data-custom-syntax-theme]) .hljs-template-variable,
+html:not([data-custom-syntax-theme]) .hljs-type,
+html:not([data-custom-syntax-theme]) .hljs-variable.language_ {
   color: var(--color-prettylights-syntax-keyword);
 }
 
-.hljs-title,
-.hljs-title.class_,
-.hljs-title.class_.inherited__,
-.hljs-title.function_ {
+html:not([data-custom-syntax-theme]) .hljs-title,
+html:not([data-custom-syntax-theme]) .hljs-title.class_,
+html:not([data-custom-syntax-theme]) .hljs-title.class_.inherited__,
+html:not([data-custom-syntax-theme]) .hljs-title.function_ {
   color: var(--color-prettylights-syntax-entity);
 }
 
-.hljs-attr,
-.hljs-attribute,
-.hljs-literal,
-.hljs-meta,
-.hljs-number,
-.hljs-operator,
-.hljs-variable,
-.hljs-selector-attr,
-.hljs-selector-class,
-.hljs-selector-id {
+html:not([data-custom-syntax-theme]) .hljs-attr,
+html:not([data-custom-syntax-theme]) .hljs-attribute,
+html:not([data-custom-syntax-theme]) .hljs-literal,
+html:not([data-custom-syntax-theme]) .hljs-meta,
+html:not([data-custom-syntax-theme]) .hljs-number,
+html:not([data-custom-syntax-theme]) .hljs-operator,
+html:not([data-custom-syntax-theme]) .hljs-variable,
+html:not([data-custom-syntax-theme]) .hljs-selector-attr,
+html:not([data-custom-syntax-theme]) .hljs-selector-class,
+html:not([data-custom-syntax-theme]) .hljs-selector-id {
   color: var(--color-prettylights-syntax-constant);
 }
 
-.hljs-regexp,
-.hljs-string,
-.hljs-meta .hljs-string {
+html:not([data-custom-syntax-theme]) .hljs-regexp,
+html:not([data-custom-syntax-theme]) .hljs-string,
+html:not([data-custom-syntax-theme]) .hljs-meta .hljs-string {
   color: var(--color-prettylights-syntax-string);
 }
 
-.hljs-built_in,
-.hljs-symbol {
+html:not([data-custom-syntax-theme]) .hljs-built_in,
+html:not([data-custom-syntax-theme]) .hljs-symbol {
   color: var(--color-prettylights-syntax-variable);
 }
 
-.hljs-comment,
-.hljs-code,
-.hljs-formula {
+html:not([data-custom-syntax-theme]) .hljs-comment,
+html:not([data-custom-syntax-theme]) .hljs-code,
+html:not([data-custom-syntax-theme]) .hljs-formula {
   color: var(--color-prettylights-syntax-comment);
 }
 
-.hljs-name,
-.hljs-quote,
-.hljs-selector-tag,
-.hljs-selector-pseudo {
+html:not([data-custom-syntax-theme]) .hljs-name,
+html:not([data-custom-syntax-theme]) .hljs-quote,
+html:not([data-custom-syntax-theme]) .hljs-selector-tag,
+html:not([data-custom-syntax-theme]) .hljs-selector-pseudo {
   color: var(--color-prettylights-syntax-entity-tag);
 }
 
-.hljs-subst {
+html:not([data-custom-syntax-theme]) .hljs-subst {
   color: var(--color-fg-default);
 }
 
-.hljs-section {
+html:not([data-custom-syntax-theme]) .hljs-section {
   color: var(--color-prettylights-syntax-markup-heading);
   font-weight: bold;
 }
 
-.hljs-bullet {
+html:not([data-custom-syntax-theme]) .hljs-bullet {
   color: var(--color-prettylights-syntax-constant);
 }
 
-.hljs-emphasis {
+html:not([data-custom-syntax-theme]) .hljs-emphasis {
   color: var(--color-fg-default);
   font-style: italic;
 }
 
-.hljs-strong {
+html:not([data-custom-syntax-theme]) .hljs-strong {
   color: var(--color-fg-default);
   font-weight: bold;
 }
 
-.hljs-addition {
+html:not([data-custom-syntax-theme]) .hljs-addition {
   color: var(--color-prettylights-syntax-markup-inserted-text);
   background-color: var(--color-prettylights-syntax-markup-inserted-bg);
 }
 
-.hljs-deletion {
+html:not([data-custom-syntax-theme]) .hljs-deletion {
   color: var(--color-prettylights-syntax-markup-deleted-text);
   background-color: var(--color-prettylights-syntax-markup-deleted-bg);
 }
@@ -14522,3 +14522,158 @@ body.release-notes-active .mobile-stats-container {
   min-height: 0;
   overflow: hidden;
 }
+
+/* ==========================================================================
+   Visual Library Settings Modal (Issue #129)
+   ========================================================================== */
+
+.visual-settings-box {
+  width: min(94vw, 560px);
+  max-width: 560px;
+}
+
+.visual-settings-body {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+  padding: 16px 20px;
+  max-height: calc(85vh - 120px);
+  overflow-y: auto;
+}
+
+.visual-setting-group {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.visual-setting-label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: var(--ui-font-md);
+  font-weight: 600;
+  color: var(--text-color);
+  margin-bottom: 2px;
+}
+
+.visual-setting-label i {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 18px;
+  height: 18px;
+  color: var(--text-secondary);
+}
+
+.visual-setting-desc {
+  font-size: var(--ui-font-sm);
+  color: var(--text-secondary);
+  line-height: 1.4;
+  margin: 0;
+}
+
+.visual-setting-select {
+  display: block;
+  width: 100%;
+  min-height: 38px;
+  padding: 8px 12px;
+  font-size: var(--ui-font-md);
+  line-height: 1.5;
+  color: var(--text-color);
+  background-color: var(--bg-color);
+  background-clip: padding-box;
+  border: 1px solid var(--border-color);
+  border-radius: 6px;
+  transition: border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
+  cursor: pointer;
+}
+
+.visual-setting-select:focus,
+.visual-setting-select:focus-visible {
+  outline: 0;
+  border-color: #0969da;
+  box-shadow: 0 0 0 3px rgba(9, 105, 218, 0.25);
+}
+
+.visual-settings-actions {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 12px 20px;
+  border-top: 1px solid var(--border-color);
+  background-color: var(--bg-color);
+  border-bottom-left-radius: inherit;
+  border-bottom-right-radius: inherit;
+}
+
+.visual-settings-actions-left {
+  display: flex;
+  align-items: center;
+}
+
+.visual-settings-actions-right {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+/* Dark mode styling for visual settings */
+[data-theme="dark"] .visual-setting-select {
+  background-color: #161b22;
+  color: #c9d1d9;
+  border-color: #30363d;
+}
+
+[data-theme="dark"] .visual-setting-select:focus,
+[data-theme="dark"] .visual-setting-select:focus-visible {
+  border-color: #58a6ff;
+  box-shadow: 0 0 0 3px rgba(56, 139, 253, 0.3);
+}
+
+[data-theme="dark"] .visual-setting-select option {
+  background-color: #161b22;
+  color: #c9d1d9;
+}
+
+[data-theme="dark"] .visual-settings-actions {
+  background-color: var(--bg-color);
+  border-top-color: #30363d;
+}
+
+/* Mobile responsiveness */
+@media (max-width: 768px) {
+  .visual-settings-box {
+    width: 96vw;
+    margin: 8px auto;
+  }
+
+  .visual-settings-body {
+    padding: 14px 16px;
+    gap: 14px;
+    max-height: calc(85vh - 140px);
+  }
+
+  .visual-setting-select {
+    min-height: 44px; /* Better touch target */
+    font-size: var(--ui-font-base, 14px);
+  }
+
+  .visual-settings-actions {
+    padding: 12px 16px;
+    flex-wrap: wrap;
+    gap: 10px;
+  }
+
+  .visual-settings-actions-left,
+  .visual-settings-actions-right {
+    width: 100%;
+  }
+
+  .visual-settings-actions-left button,
+  .visual-settings-actions-right button {
+    width: 100%;
+    min-height: 40px;
+    justify-content: center;
+  }
+}
diff --git a/index.html b/index.html
index 180b6e85..8eab72b1 100644
--- a/index.html
+++ b/index.html
@@ -14,6 +14,8 @@
     <!-- PERF-015: Preload critical-path resources for faster discovery -->
     <link rel="preload" href="styles.css" as="style">
     <link rel="preload" href="workspace-storage.js" as="script">
+    <link rel="preload" href="visual-settings-adapters.js" as="script">
+    <link rel="preload" href="visual-settings-store.js" as="script">
     <link rel="preload" href="script.js" as="script">
 
     <!-- Canonical Link -->
@@ -218,6 +220,10 @@ <h1 class="h4 mb-0">Markdown Viewer<span class="visually-hidden"> - Online Markd
                         <i class="lucide lucide-info" aria-hidden="true"></i>
                     </button>
 
+                    <button id="visual-settings-button" class="header-icon-command" type="button" title="Visual library settings" aria-label="Open visual library settings">
+                        <i class="lucide lucide-layers-3" aria-hidden="true"></i>
+                    </button>
+
                     <div class="dropdown settings-dropdown">
                         <button class="header-icon-command" type="button" id="workspaceSettingsDropdown" data-bs-toggle="dropdown" data-bs-auto-close="outside" aria-expanded="false" title="Workspace settings (Ctrl/Cmd+,)" aria-label="Open workspace settings" aria-keyshortcuts="Control+, Meta+,">
                             <i class="lucide lucide-settings" aria-hidden="true"></i>
@@ -229,6 +235,9 @@ <h1 class="h4 mb-0">Markdown Viewer<span class="visually-hidden"> - Online Markd
                             <button id="theme-toggle" class="settings-menu-item app-menu-item settings-menu-item--toggle" type="button" aria-pressed="false" title="Toggle appearance">
                                 <i id="theme-toggle-icon" class="lucide lucide-sun" aria-hidden="true"></i><span class="app-menu-copy"><span class="app-menu-label">Appearance</span><span id="theme-toggle-description" class="app-menu-description">Switch to dark appearance</span></span><span class="settings-switch themed-settings-switch" aria-hidden="true"><span><i id="theme-switch-icon" class="lucide lucide-sun settings-switch-icon"></i></span></span>
                             </button>
+                            <button id="menu-visual-settings-button" class="settings-menu-item app-menu-item" type="button" title="Visual library settings">
+                                <i class="lucide lucide-layers-3" aria-hidden="true"></i><span class="app-menu-copy"><span class="app-menu-label">Visual Settings</span><span class="app-menu-description">Themes, fonts, and styles for libraries</span></span>
+                            </button>
                             <div class="dropdown dropstart settings-language-dropdown">
                                 <button class="settings-menu-item app-menu-item dropdown-toggle" type="button" id="languageDropdown" data-bs-toggle="dropdown" data-bs-auto-close="outside" aria-expanded="false" title="Switch language">
                                     <i class="lucide lucide-languages" aria-hidden="true"></i><span class="app-menu-copy"><span id="current-lang-label" class="app-menu-label">English</span><span class="app-menu-description">Application language</span></span>
@@ -413,6 +422,9 @@ <h3 class="mobile-menu-group-label">Actions</h3>
                                         <button id="mobile-private-mode-toggle" class="mobile-menu-item mobile-settings-item mobile-settings-toggle" type="button" aria-pressed="false" title="Turn private mode on">
                                             <span class="mobile-menu-item-label"><i class="lucide lucide-hat-glasses" aria-hidden="true"></i><span>Private mode</span></span><span class="settings-switch themed-settings-switch" aria-hidden="true"><span></span></span>
                                         </button>
+                                        <button id="mobile-visual-settings-button" class="mobile-menu-item mobile-settings-item" type="button" title="Visual library settings">
+                                            <span class="mobile-menu-item-label"><i class="lucide lucide-layers-3" aria-hidden="true"></i><span>Visual Settings</span></span>
+                                        </button>
                                         <button id="mobile-storage-settings-button" class="mobile-menu-item mobile-settings-item" type="button" title="Storage and backup">
                                             <span class="mobile-menu-item-label"><i class="lucide lucide-box" aria-hidden="true"></i><span>Storage and Backup</span></span>
                                         </button>
@@ -656,6 +668,83 @@ <h2 id="document-sidebar-title">Explorer</h2>
         </div>
         <input id="storage-backup-file-input" type="file" accept=".zip,application/zip" hidden>
 
+        <!-- Visual Library Settings Modal -->
+        <div id="visual-settings-modal" class="reset-modal-overlay modal-overlay" role="dialog" aria-modal="true" aria-labelledby="visual-settings-title" aria-hidden="true" style="display:none;">
+          <div class="reset-modal-box reset-modal-box--wide app-modal-box modal-box visual-settings-box">
+            <div class="modal-header">
+              <p id="visual-settings-title" class="reset-modal-message">Visual Library Settings</p>
+              <button type="button" class="modal-close-btn" id="visual-settings-close-icon" aria-label="Close visual library settings">
+                <i class="lucide lucide-x" aria-hidden="true"></i>
+              </button>
+            </div>
+            <div class="modal-body visual-settings-body">
+              <p class="visual-setting-desc mb-2">Configure default themes, fonts, and rendering styles for preview visual libraries.</p>
+
+              <!-- highlight.js theme -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-hljs-theme" class="visual-setting-label">
+                  <i class="lucide lucide-code-2" aria-hidden="true"></i> Syntax Highlighting Theme (highlight.js)
+                </label>
+                <select id="visual-setting-hljs-theme" class="form-select visual-setting-select" aria-describedby="visual-setting-hljs-desc">
+                </select>
+                <p id="visual-setting-hljs-desc" class="visual-setting-desc">Color palette for highlighted code blocks in the preview.</p>
+              </div>
+
+              <!-- MathJax math font -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-math-font" class="visual-setting-label">
+                  <i class="lucide lucide-spline" aria-hidden="true"></i> Math Font (MathJax)
+                </label>
+                <select id="visual-setting-math-font" class="form-select visual-setting-select" aria-describedby="visual-setting-math-desc">
+                </select>
+                <p id="visual-setting-math-desc" class="visual-setting-desc">Font family used for rendering LaTeX mathematical notation.</p>
+              </div>
+
+              <!-- Mermaid theme -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-mermaid-theme" class="visual-setting-label">
+                  <i class="lucide lucide-workflow" aria-hidden="true"></i> Diagram Theme (Mermaid)
+                </label>
+                <select id="visual-setting-mermaid-theme" class="form-select visual-setting-select" aria-describedby="visual-setting-mermaid-theme-desc">
+                </select>
+                <p id="visual-setting-mermaid-theme-desc" class="visual-setting-desc">Color palette and appearance for Mermaid diagrams.</p>
+              </div>
+
+              <!-- Mermaid look -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-mermaid-look" class="visual-setting-label">
+                  <i class="lucide lucide-square-pen" aria-hidden="true"></i> Diagram Drawing Style (Mermaid Look)
+                </label>
+                <select id="visual-setting-mermaid-look" class="form-select visual-setting-select" aria-describedby="visual-setting-mermaid-look-desc">
+                </select>
+                <p id="visual-setting-mermaid-look-desc" class="visual-setting-desc">Geometric drawing style (Classic or Hand Drawn sketch).</p>
+              </div>
+
+              <!-- JoyPixels emoji skin tone -->
+              <div class="visual-setting-group">
+                <label for="visual-setting-emoji-skin-tone" class="visual-setting-label">
+                  <i class="lucide lucide-smile" aria-hidden="true"></i> Emoji Skin Tone (JoyPixels)
+                </label>
+                <select id="visual-setting-emoji-skin-tone" class="form-select visual-setting-select" aria-describedby="visual-setting-emoji-skin-tone-desc">
+                </select>
+                <p id="visual-setting-emoji-skin-tone-desc" class="visual-setting-desc">Default Fitzpatrick skin tone for supported emoji shortcodes.</p>
+              </div>
+            </div>
+            <div class="reset-modal-actions visual-settings-actions">
+              <div class="visual-settings-actions-left">
+                <button type="button" class="reset-modal-btn reset-modal-cancel" id="visual-settings-reset-btn" title="Reset all visual settings to defaults">
+                  <i class="lucide lucide-refresh-cw me-1" aria-hidden="true"></i> Reset to defaults
+                </button>
+              </div>
+              <div class="visual-settings-actions-right">
+                <button type="button" class="reset-modal-btn storage-primary-action" id="visual-settings-close-btn">
+                  Close
+                </button>
+              </div>
+            </div>
+          </div>
+        </div>
+
         <!-- Trash Modal -->
         <div id="trash-modal" class="reset-modal-overlay modal-overlay" role="dialog" aria-modal="true" aria-labelledby="trash-modal-title" aria-describedby="trash-modal-description" aria-hidden="true" style="display:none;">
           <div class="reset-modal-box reset-modal-box--wide app-modal-box modal-box trash-modal-box">
@@ -2309,6 +2398,8 @@ <h3>No comments yet</h3>
     <script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap/5.3.2/js/bootstrap.bundle.min.js" integrity="sha384-C6RzsynM9kWDrMNeT87bh95OGNyZPhcTNXj1NW7RuBCsyN/o0jlpcV8Qyq46cDfL" crossorigin="anonymous" defer></script>
     <script src="assets/document-toc.js" defer></script>
     <script src="workspace-storage.js" defer></script>
+    <script src="visual-settings-adapters.js" defer></script>
+    <script src="visual-settings-store.js" defer></script>
     <script src="script.js" defer></script>
     <!-- Screen reader dynamic accessibility announcer -->
     <div id="app-accessibility-announcer" class="visually-hidden" aria-live="polite" aria-atomic="true"></div>
diff --git a/script.js b/script.js
index 4c25a960..b775309c 100644
--- a/script.js
+++ b/script.js
@@ -111,6 +111,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // PERF-002: Lazy script loader for optional heavy libraries
   const CDN_INTEGRITY = Object.freeze({
     'https://cdn.jsdelivr.net/npm/mermaid@11.15.0/dist/mermaid.min.js': 'sha384-yQ4mmBBT+vhTAwjFH0toJXNYJ6O4usWnt6EPIdWwrRvx2V/n5lXuDZQwQFeSFydF',
+    'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js': 'sha384-2f5bAKjuFIbbQ+0O9aSu5W1OyLA4q80QrDEvjXDwJexjU9VqoJhMgw19pYVpSJab',
     'https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js': 'sha384-M5jmNxKC9EVnuqeMwRHvFuYUE8Hhp0TgBruj/GZRkYtiMrCRgH7yvv5KY+Owi7TW',
     'https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.5.1/jspdf.umd.min.js': 'sha384-JcnsjUPPylna1s1fvi1u12X5qjY5OL56iySh75FdtrwhO/SWXgMjoVqcKyIIWOLk',
     'https://cdnjs.cloudflare.com/ajax/libs/html2canvas/1.4.1/html2canvas.min.js': 'sha384-ZZ1pncU3bQe8y31yfZdMFdSpttDoPmOZg2wguVK9almUodir1PghgT0eY7Mrty8H',
@@ -188,7 +189,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // CDN URLs for lazy-loaded libraries
   const CDN = {
     mermaid: 'https://cdn.jsdelivr.net/npm/mermaid@11.15.0/dist/mermaid.min.js',
-    mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js',
+    mathjax: 'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js',
     jspdf: 'https://cdnjs.cloudflare.com/ajax/libs/jspdf/2.5.1/jspdf.umd.min.js',
     html2canvas: 'https://cdnjs.cloudflare.com/ajax/libs/html2canvas/1.4.1/html2canvas.min.js',
     pako: 'https://cdnjs.cloudflare.com/ajax/libs/pako/2.1.0/pako.min.js',
@@ -211,7 +212,7 @@ document.addEventListener("DOMContentLoaded", async function () {
   // Resolve local paths for desktop (Neutralinojs) offline support
   if (typeof Neutralino !== 'undefined') {
     CDN.mermaid = '/libs/mermaid.min.js';
-    CDN.mathjax = '/libs/tex-mml-chtml.min.js';
+    CDN.mathjax = '/libs/tex-mml-chtml.js';
     CDN.jspdf = '/libs/jspdf.umd.min.js';
     CDN.html2canvas = '/libs/html2canvas.min.js';
     CDN.pako = '/libs/pako.min.js';
@@ -943,6 +944,18 @@ document.addEventListener("DOMContentLoaded", async function () {
   const aboutVersion = document.getElementById("about-version");
   const aboutReleaseNotes = document.getElementById("about-release-notes");
   const privateModeToggle = document.getElementById("private-mode-toggle");
+  const visualSettingsButton = document.getElementById("visual-settings-button");
+  const menuVisualSettingsButton = document.getElementById("menu-visual-settings-button");
+  const mobileVisualSettingsButton = document.getElementById("mobile-visual-settings-button");
+  const visualSettingsModal = document.getElementById("visual-settings-modal");
+  const visualSettingsCloseBtn = document.getElementById("visual-settings-close-btn");
+  const visualSettingsCloseIcon = document.getElementById("visual-settings-close-icon");
+  const visualSettingsResetBtn = document.getElementById("visual-settings-reset-btn");
+  const selectHljsTheme = document.getElementById("visual-setting-hljs-theme");
+  const selectMathFont = document.getElementById("visual-setting-math-font");
+  const selectMermaidTheme = document.getElementById("visual-setting-mermaid-theme");
+  const selectMermaidLook = document.getElementById("visual-setting-mermaid-look");
+  const selectEmojiSkinTone = document.getElementById("visual-setting-emoji-skin-tone");
   const storageSettingsButton = document.getElementById("storage-settings-button");
   const trashSettingsButton = document.getElementById("trash-settings-button");
   const storageSettingsModal = document.getElementById("storage-settings-modal");
@@ -2237,22 +2250,28 @@ document.addEventListener("DOMContentLoaded", async function () {
   applyDirectionToContent(initialDirection);
   updateDirectionToggleUI(initialDirection);
 
-  // Track last Mermaid theme to avoid redundant re-initialization (PERF-005)
+  // Track last Mermaid theme and look to avoid redundant re-initialization (PERF-005)
   let _lastMermaidTheme = null;
   let _mermaidThemeReinitTimeout = null;
   let _themeTransitionTimeout = null;
   const initMermaid = (forceReinit, themeOverride) => {
     if (typeof mermaid === 'undefined') return; // PERF-002: Not loaded yet
     const currentTheme = themeOverride || document.documentElement.getAttribute("data-theme");
-    const mermaidTheme = currentTheme === "dark" ? "dark" : "default";
+    const config = window.visualSettingsAdapters
+      ? window.visualSettingsAdapters.getMermaidConfig(currentTheme)
+      : { theme: currentTheme === "dark" ? "dark" : "default", look: "classic" };
+    const mermaidTheme = config.theme;
+    const mermaidLook = config.look || "classic";
     
-    // Skip re-initialization if theme hasn't changed (PERF-005)
-    if (!forceReinit && _lastMermaidTheme === mermaidTheme) return;
-    _lastMermaidTheme = mermaidTheme;
+    // Skip re-initialization if theme & look haven't changed (PERF-005)
+    const configKey = `${mermaidTheme}:${mermaidLook}`;
+    if (!forceReinit && _lastMermaidTheme === configKey) return;
+    _lastMermaidTheme = configKey;
     
     mermaid.initialize({
       startOnLoad: false,
       theme: mermaidTheme,
+      look: mermaidLook,
       securityLevel: 'strict',
       flowchart: { useMaxWidth: true, htmlLabels: true },
       fontSize: 16,
@@ -4108,7 +4127,18 @@ document.addEventListener("DOMContentLoaded", async function () {
 
   function configureMathJax() {
     if (window.MathJax && typeof MathJax.typesetPromise === 'function') return;
-    if (window.MathJax && MathJax.loader && MathJax.tex) return;
+    if (window.MathJax && MathJax.loader && MathJax.tex) {
+      if (window.MathJax.chtml && typeof window.MathJax.chtml.font === 'string') {
+        delete window.MathJax.chtml.font;
+      }
+      return;
+    }
+    const initialFont = window.visualSettingsAdapters
+      ? window.visualSettingsAdapters.getCurrentMathFont()
+      : 'mathjax-modern';
+    if (window.MathJax && window.MathJax.chtml && typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
     window.MathJax = {
       startup: {
         typeset: false
@@ -4116,6 +4146,9 @@ document.addEventListener("DOMContentLoaded", async function () {
       options: {
         a11y: { inTabOrder: false }
       },
+      output: {
+        font: initialFont
+      },
       tex: {
         inlineMath: [['$', '$'], ['\\(', '\\)']],
         displayMath: [['$$', '$$'], ['\\[', '\\]']],
@@ -4133,7 +4166,15 @@ document.addEventListener("DOMContentLoaded", async function () {
 
     configureMathJax();
     if (!mathJaxLoadPromise) {
-      mathJaxLoadPromise = loadScript(CDN.mathjax)
+      const initialFont = window.visualSettingsAdapters
+        ? window.visualSettingsAdapters.getCurrentMathFont()
+        : 'mathjax-modern';
+      const fontPreloadPromise = (initialFont && initialFont !== 'mathjax-modern' && window.visualSettingsAdapters)
+        ? window.visualSettingsAdapters.applyMathFont(initialFont)
+        : Promise.resolve();
+
+      mathJaxLoadPromise = fontPreloadPromise
+        .then(function() { return loadScript(CDN.mathjax); })
         .then(function() {
           return MathJax.startup && MathJax.startup.promise
             ? MathJax.startup.promise
@@ -15014,6 +15055,7 @@ ${selector} .arrowheadPath {
   }
 
   function renderMarkdown(options) {
+    window.renderMarkdown = renderMarkdown;
     stopActiveAbcPlayback();
     options = options || {};
     const rawVal = markdownEditor.value;
@@ -17058,7 +17100,10 @@ ${selector} .arrowheadPath {
       
       while ((match = emojiRegex.exec(text)) !== null) {
         const shortcode = match[1];
-        const emoji = joypixels.shortnameToUnicode(`:${shortcode}:`);
+        const effectiveShortcode = (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.resolveEmojiShortcode === 'function')
+          ? window.visualSettingsAdapters.resolveEmojiShortcode(shortcode)
+          : shortcode;
+        const emoji = joypixels.shortnameToUnicode(`:${effectiveShortcode}:`);
         
         if (emoji !== `:${shortcode}:`) { // If conversion was successful
           hasEmoji = true;
@@ -24031,6 +24076,13 @@ ${selector} .arrowheadPath {
     });
   }
 
+  if (mobileVisualSettingsButton) {
+    mobileVisualSettingsButton.addEventListener('click', function() {
+      closeMobileMenu();
+      openVisualSettingsModal(mobileVisualSettingsButton);
+    });
+  }
+
   if (mobileStorageSettingsButton) {
     mobileStorageSettingsButton.addEventListener('click', function() {
       closeMobileMenu();
@@ -24045,6 +24097,179 @@ ${selector} .arrowheadPath {
     });
   }
 
+  // Visual Library Settings (Issue #129)
+  function populateVisualSettingsOptions() {
+    const options = (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getOptions === 'function')
+      ? window.visualSettingsAdapters.getOptions()
+      : (window.visualSettingsStore ? window.visualSettingsStore.DEFAULT_OPTIONS : null);
+
+    if (!options) return;
+
+    const list = [
+      { el: selectHljsTheme, opts: options.hljsTheme },
+      { el: selectMathFont, opts: options.mathFont },
+      { el: selectMermaidTheme, opts: options.mermaidTheme },
+      { el: selectMermaidLook, opts: options.mermaidLook },
+      { el: selectEmojiSkinTone, opts: options.emojiSkinTone }
+    ];
+
+    list.forEach(function(item) {
+      if (!item.el || !item.opts) return;
+      const currentVal = item.el.value;
+      item.el.innerHTML = '';
+      item.opts.forEach(function(opt) {
+        const val = typeof opt === 'object' && opt !== null ? opt.value : opt;
+        const label = typeof opt === 'object' && opt !== null ? opt.label : opt;
+        const optEl = document.createElement('option');
+        optEl.value = val;
+        optEl.textContent = label;
+        item.el.appendChild(optEl);
+      });
+      if (currentVal) item.el.value = currentVal;
+    });
+  }
+
+  function syncVisualSettingsUI() {
+    if (!window.visualSettingsStore) return;
+    const settings = window.visualSettingsStore.getSettings();
+    if (selectHljsTheme && settings.hljsTheme) selectHljsTheme.value = settings.hljsTheme;
+    if (selectMathFont && settings.mathFont) selectMathFont.value = settings.mathFont;
+    if (selectMermaidTheme && settings.mermaidTheme) selectMermaidTheme.value = settings.mermaidTheme;
+    if (selectMermaidLook && settings.mermaidLook) selectMermaidLook.value = settings.mermaidLook;
+    if (selectEmojiSkinTone && settings.emojiSkinTone) selectEmojiSkinTone.value = settings.emojiSkinTone;
+  }
+
+  function openVisualSettingsModal(opener) {
+    if (!visualSettingsModal) return;
+    populateVisualSettingsOptions();
+    syncVisualSettingsUI();
+    openAppModal(visualSettingsModal, {
+      focusTarget: selectHljsTheme,
+      returnFocus: opener || document.activeElement,
+      onClose: closeVisualSettingsModal
+    });
+  }
+
+  function closeVisualSettingsModal() {
+    if (visualSettingsModal) closeAppModal(visualSettingsModal);
+  }
+
+  let _visualSettingsDebounceTimer = null;
+
+  async function applyVisualSettings(settings, changedKey) {
+    if (window.visualSettingsAdapters) {
+      try {
+        if (changedKey === 'hljsTheme') {
+          await window.visualSettingsAdapters.applyHighlightTheme(settings.hljsTheme);
+        } else if (changedKey === 'mathFont') {
+          await window.visualSettingsAdapters.applyMathFont(settings.mathFont);
+        } else if (changedKey === 'mermaidTheme' || changedKey === 'mermaidLook') {
+          await window.visualSettingsAdapters.applyMermaid({
+            theme: settings.mermaidTheme,
+            look: settings.mermaidLook
+          });
+        } else if (changedKey === 'emojiSkinTone') {
+          await window.visualSettingsAdapters.applyEmojiSkinTone(settings.emojiSkinTone);
+        } else {
+          if (typeof window.visualSettingsAdapters.applyAll === 'function') {
+            await window.visualSettingsAdapters.applyAll(settings);
+          } else {
+            await Promise.all([
+              window.visualSettingsAdapters.applyHighlightTheme && window.visualSettingsAdapters.applyHighlightTheme(settings.hljsTheme),
+              window.visualSettingsAdapters.applyMathFont && window.visualSettingsAdapters.applyMathFont(settings.mathFont),
+              window.visualSettingsAdapters.applyMermaid && window.visualSettingsAdapters.applyMermaid({ theme: settings.mermaidTheme, look: settings.mermaidLook }),
+              window.visualSettingsAdapters.applyEmojiSkinTone && window.visualSettingsAdapters.applyEmojiSkinTone(settings.emojiSkinTone)
+            ]);
+          }
+        }
+      } catch (err) {
+        console.warn('Error applying visual setting via adapter:', err);
+      }
+    }
+
+    // Invalidate secondary split document preview cache (P6)
+    documentSplitPreviewContent = null;
+    if (secondarySplitTabId && typeof renderDocumentSplitView === 'function') {
+      renderDocumentSplitView();
+    }
+
+    // Check if re-render is needed according to adapter contract (e.g. hljsTheme does not need markdown AST re-render)
+    const needsRender = !window.visualSettingsAdapters ||
+      typeof window.visualSettingsAdapters.needsRerender !== 'function' ||
+      window.visualSettingsAdapters.needsRerender(changedKey);
+
+    if (needsRender) {
+      if (_visualSettingsDebounceTimer) {
+        clearTimeout(_visualSettingsDebounceTimer);
+      }
+      _visualSettingsDebounceTimer = setTimeout(function() {
+        _visualSettingsDebounceTimer = null;
+        renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+      }, 50);
+    }
+  }
+
+  if (visualSettingsButton) {
+    visualSettingsButton.addEventListener('click', function() {
+      openVisualSettingsModal(visualSettingsButton);
+    });
+  }
+
+  if (menuVisualSettingsButton) {
+    menuVisualSettingsButton.addEventListener('click', function() {
+      const settingsToggle = document.getElementById('workspaceSettingsDropdown');
+      if (settingsToggle && window.bootstrap && bootstrap.Dropdown) {
+        bootstrap.Dropdown.getOrCreateInstance(settingsToggle).hide();
+      }
+      openVisualSettingsModal(menuVisualSettingsButton);
+    });
+  }
+
+  if (visualSettingsCloseBtn) visualSettingsCloseBtn.addEventListener('click', closeVisualSettingsModal);
+  if (visualSettingsCloseIcon) visualSettingsCloseIcon.addEventListener('click', closeVisualSettingsModal);
+
+  if (selectHljsTheme) {
+    selectHljsTheme.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('hljsTheme', selectHljsTheme.value);
+    });
+  }
+  if (selectMathFont) {
+    selectMathFont.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mathFont', selectMathFont.value);
+    });
+  }
+  if (selectMermaidTheme) {
+    selectMermaidTheme.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mermaidTheme', selectMermaidTheme.value);
+    });
+  }
+  if (selectMermaidLook) {
+    selectMermaidLook.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('mermaidLook', selectMermaidLook.value);
+    });
+  }
+  if (selectEmojiSkinTone) {
+    selectEmojiSkinTone.addEventListener('change', function() {
+      if (window.visualSettingsStore) window.visualSettingsStore.updateSetting('emojiSkinTone', selectEmojiSkinTone.value);
+    });
+  }
+
+  if (visualSettingsResetBtn) {
+    visualSettingsResetBtn.addEventListener('click', function() {
+      if (window.visualSettingsStore) {
+        window.visualSettingsStore.resetSettings();
+        syncVisualSettingsUI();
+      }
+    });
+  }
+
+  if (window.visualSettingsStore) {
+    window.visualSettingsStore.onSettingsChange(function(settings, changedKey) {
+      syncVisualSettingsUI();
+      applyVisualSettings(settings, changedKey);
+    });
+  }
+
   if (storageSettingsButton) {
     storageSettingsButton.addEventListener('click', function() {
       const settingsToggle = document.getElementById('workspaceSettingsDropdown');
@@ -24525,8 +24750,19 @@ ${selector} .arrowheadPath {
       }
     }
 
+    if (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.onAppThemeChange === 'function') {
+      window.visualSettingsAdapters.onAppThemeChange(theme);
+    }
+
     updateMapThemes();
     updateStlThemes();
+    if (window.visualSettingsAdapters && typeof window.visualSettingsAdapters.onAppThemeChange === 'function') {
+      try {
+        window.visualSettingsAdapters.onAppThemeChange(theme);
+      } catch (err) {
+        console.warn('Error in visualSettingsAdapters.onAppThemeChange:', err);
+      }
+    }
   });
 
   async function nativeSaveMarkdown() {
@@ -32303,6 +32539,16 @@ ${selector} .arrowheadPath {
       activate: releaseNotesLaunch.activate
     });
   }
+
+  // Initialize Visual Library Settings (Issue #129)
+  window.renderMarkdown = renderMarkdown;
+  populateVisualSettingsOptions();
+  syncVisualSettingsUI();
+  if (window.visualSettingsStore) {
+    const initialVisualSettings = window.visualSettingsStore.getSettings();
+    await applyVisualSettings(initialVisualSettings, '*');
+  }
+
   document.documentElement.dataset.appReady = 'true';
   window.dispatchEvent(new Event('markdown-viewer:ready'));
 });
diff --git a/seo/generate-sitemap.mjs b/seo/generate-sitemap.mjs
index 2ea8c5d8..345e40bd 100644
--- a/seo/generate-sitemap.mjs
+++ b/seo/generate-sitemap.mjs
@@ -32,8 +32,9 @@ export function buildSitemap() {
 if (process.argv[1] && path.resolve(process.argv[1]) === modulePath) {
   const expected = buildSitemap();
   if (process.argv.includes('--check')) {
-    const actual = fs.readFileSync(sitemapPath, 'utf8');
-    if (actual !== expected) {
+    const actual = fs.readFileSync(sitemapPath, 'utf8').replace(/\r\n/g, '\n');
+    const normalizedExpected = expected.replace(/\r\n/g, '\n');
+    if (actual !== normalizedExpected) {
       throw new Error('sitemap.xml is stale. Run: npm run seo:sitemap');
     }
     console.log('SEO sitemap is current.');
diff --git a/styles.css b/styles.css
index 5671d94c..afce7f22 100644
--- a/styles.css
+++ b/styles.css
@@ -4112,94 +4112,94 @@ a:focus {
   background-color: var(--code-bg);
 }
 
-/* Syntax Highlighting Mapping to GitHub Variables */
-.hljs {
+/* Syntax Highlighting Mapping to GitHub Variables (scoped to when no external syntax theme is selected) */
+html:not([data-custom-syntax-theme]) .hljs {
   color: var(--color-fg-default);
 }
 
-.hljs-doctag,
-.hljs-keyword,
-.hljs-meta .hljs-keyword,
-.hljs-template-tag,
-.hljs-template-variable,
-.hljs-type,
-.hljs-variable.language_ {
+html:not([data-custom-syntax-theme]) .hljs-doctag,
+html:not([data-custom-syntax-theme]) .hljs-keyword,
+html:not([data-custom-syntax-theme]) .hljs-meta .hljs-keyword,
+html:not([data-custom-syntax-theme]) .hljs-template-tag,
+html:not([data-custom-syntax-theme]) .hljs-template-variable,
+html:not([data-custom-syntax-theme]) .hljs-type,
+html:not([data-custom-syntax-theme]) .hljs-variable.language_ {
   color: var(--color-prettylights-syntax-keyword);
 }
 
-.hljs-title,
-.hljs-title.class_,
-.hljs-title.class_.inherited__,
-.hljs-title.function_ {
+html:not([data-custom-syntax-theme]) .hljs-title,
+html:not([data-custom-syntax-theme]) .hljs-title.class_,
+html:not([data-custom-syntax-theme]) .hljs-title.class_.inherited__,
+html:not([data-custom-syntax-theme]) .hljs-title.function_ {
   color: var(--color-prettylights-syntax-entity);
 }
 
-.hljs-attr,
-.hljs-attribute,
-.hljs-literal,
-.hljs-meta,
-.hljs-number,
-.hljs-operator,
-.hljs-variable,
-.hljs-selector-attr,
-.hljs-selector-class,
-.hljs-selector-id {
+html:not([data-custom-syntax-theme]) .hljs-attr,
+html:not([data-custom-syntax-theme]) .hljs-attribute,
+html:not([data-custom-syntax-theme]) .hljs-literal,
+html:not([data-custom-syntax-theme]) .hljs-meta,
+html:not([data-custom-syntax-theme]) .hljs-number,
+html:not([data-custom-syntax-theme]) .hljs-operator,
+html:not([data-custom-syntax-theme]) .hljs-variable,
+html:not([data-custom-syntax-theme]) .hljs-selector-attr,
+html:not([data-custom-syntax-theme]) .hljs-selector-class,
+html:not([data-custom-syntax-theme]) .hljs-selector-id {
   color: var(--color-prettylights-syntax-constant);
 }
 
-.hljs-regexp,
-.hljs-string,
-.hljs-meta .hljs-string {
+html:not([data-custom-syntax-theme]) .hljs-regexp,
+html:not([data-custom-syntax-theme]) .hljs-string,
+html:not([data-custom-syntax-theme]) .hljs-meta .hljs-string {
   color: var(--color-prettylights-syntax-string);
 }
 
-.hljs-built_in,
-.hljs-symbol {
+html:not([data-custom-syntax-theme]) .hljs-built_in,
+html:not([data-custom-syntax-theme]) .hljs-symbol {
   color: var(--color-prettylights-syntax-variable);
 }
 
-.hljs-comment,
-.hljs-code,
-.hljs-formula {
+html:not([data-custom-syntax-theme]) .hljs-comment,
+html:not([data-custom-syntax-theme]) .hljs-code,
+html:not([data-custom-syntax-theme]) .hljs-formula {
   color: var(--color-prettylights-syntax-comment);
 }
 
-.hljs-name,
-.hljs-quote,
-.hljs-selector-tag,
-.hljs-selector-pseudo {
+html:not([data-custom-syntax-theme]) .hljs-name,
+html:not([data-custom-syntax-theme]) .hljs-quote,
+html:not([data-custom-syntax-theme]) .hljs-selector-tag,
+html:not([data-custom-syntax-theme]) .hljs-selector-pseudo {
   color: var(--color-prettylights-syntax-entity-tag);
 }
 
-.hljs-subst {
+html:not([data-custom-syntax-theme]) .hljs-subst {
   color: var(--color-fg-default);
 }
 
-.hljs-section {
+html:not([data-custom-syntax-theme]) .hljs-section {
   color: var(--color-prettylights-syntax-markup-heading);
   font-weight: bold;
 }
 
-.hljs-bullet {
+html:not([data-custom-syntax-theme]) .hljs-bullet {
   color: var(--color-prettylights-syntax-constant);
 }
 
-.hljs-emphasis {
+html:not([data-custom-syntax-theme]) .hljs-emphasis {
   color: var(--color-fg-default);
   font-style: italic;
 }
 
-.hljs-strong {
+html:not([data-custom-syntax-theme]) .hljs-strong {
   color: var(--color-fg-default);
   font-weight: bold;
 }
 
-.hljs-addition {
+html:not([data-custom-syntax-theme]) .hljs-addition {
   color: var(--color-prettylights-syntax-markup-inserted-text);
   background-color: var(--color-prettylights-syntax-markup-inserted-bg);
 }
 
-.hljs-deletion {
+html:not([data-custom-syntax-theme]) .hljs-deletion {
   color: var(--color-prettylights-syntax-markup-deleted-text);
   background-color: var(--color-prettylights-syntax-markup-deleted-bg);
 }
@@ -14522,3 +14522,158 @@ body.release-notes-active .mobile-stats-container {
   min-height: 0;
   overflow: hidden;
 }
+
+/* ==========================================================================
+   Visual Library Settings Modal (Issue #129)
+   ========================================================================== */
+
+.visual-settings-box {
+  width: min(94vw, 560px);
+  max-width: 560px;
+}
+
+.visual-settings-body {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+  padding: 16px 20px;
+  max-height: calc(85vh - 120px);
+  overflow-y: auto;
+}
+
+.visual-setting-group {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.visual-setting-label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: var(--ui-font-md);
+  font-weight: 600;
+  color: var(--text-color);
+  margin-bottom: 2px;
+}
+
+.visual-setting-label i {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 18px;
+  height: 18px;
+  color: var(--text-secondary);
+}
+
+.visual-setting-desc {
+  font-size: var(--ui-font-sm);
+  color: var(--text-secondary);
+  line-height: 1.4;
+  margin: 0;
+}
+
+.visual-setting-select {
+  display: block;
+  width: 100%;
+  min-height: 38px;
+  padding: 8px 12px;
+  font-size: var(--ui-font-md);
+  line-height: 1.5;
+  color: var(--text-color);
+  background-color: var(--bg-color);
+  background-clip: padding-box;
+  border: 1px solid var(--border-color);
+  border-radius: 6px;
+  transition: border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
+  cursor: pointer;
+}
+
+.visual-setting-select:focus,
+.visual-setting-select:focus-visible {
+  outline: 0;
+  border-color: #0969da;
+  box-shadow: 0 0 0 3px rgba(9, 105, 218, 0.25);
+}
+
+.visual-settings-actions {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 12px 20px;
+  border-top: 1px solid var(--border-color);
+  background-color: var(--bg-color);
+  border-bottom-left-radius: inherit;
+  border-bottom-right-radius: inherit;
+}
+
+.visual-settings-actions-left {
+  display: flex;
+  align-items: center;
+}
+
+.visual-settings-actions-right {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+/* Dark mode styling for visual settings */
+[data-theme="dark"] .visual-setting-select {
+  background-color: #161b22;
+  color: #c9d1d9;
+  border-color: #30363d;
+}
+
+[data-theme="dark"] .visual-setting-select:focus,
+[data-theme="dark"] .visual-setting-select:focus-visible {
+  border-color: #58a6ff;
+  box-shadow: 0 0 0 3px rgba(56, 139, 253, 0.3);
+}
+
+[data-theme="dark"] .visual-setting-select option {
+  background-color: #161b22;
+  color: #c9d1d9;
+}
+
+[data-theme="dark"] .visual-settings-actions {
+  background-color: var(--bg-color);
+  border-top-color: #30363d;
+}
+
+/* Mobile responsiveness */
+@media (max-width: 768px) {
+  .visual-settings-box {
+    width: 96vw;
+    margin: 8px auto;
+  }
+
+  .visual-settings-body {
+    padding: 14px 16px;
+    gap: 14px;
+    max-height: calc(85vh - 140px);
+  }
+
+  .visual-setting-select {
+    min-height: 44px; /* Better touch target */
+    font-size: var(--ui-font-base, 14px);
+  }
+
+  .visual-settings-actions {
+    padding: 12px 16px;
+    flex-wrap: wrap;
+    gap: 10px;
+  }
+
+  .visual-settings-actions-left,
+  .visual-settings-actions-right {
+    width: 100%;
+  }
+
+  .visual-settings-actions-left button,
+  .visual-settings-actions-right button {
+    width: 100%;
+    min-height: 40px;
+    justify-content: center;
+  }
+}
diff --git a/sw.js b/sw.js
index c8ac6549..8fb37a14 100644
--- a/sw.js
+++ b/sw.js
@@ -6,6 +6,8 @@ const CRITICAL_ASSETS = [
   './',
   './index.html',
   './workspace-storage.js',
+  './visual-settings-adapters.js',
+  './visual-settings-store.js',
   './script.js',
   './preview-worker.js',
   './styles.css',
@@ -31,6 +33,8 @@ const NETWORK_FIRST_LOCAL_PATHS = new Set([
   '/',
   '/index.html',
   '/workspace-storage.js',
+  '/visual-settings-adapters.js',
+  '/visual-settings-store.js',
   '/script.js',
   '/preview-worker.js',
   '/styles.css',
diff --git a/tests/e2e/visual-settings.spec.js b/tests/e2e/visual-settings.spec.js
new file mode 100644
index 00000000..c3b7a7ba
--- /dev/null
+++ b/tests/e2e/visual-settings.spec.js
@@ -0,0 +1,841 @@
+const { test, expect } = require('@playwright/test');
+const { openApp, waitForAppReady } = require('../helpers/app.js');
+
+test.describe('Visual Library Settings Panel (Issue #129)', () => {
+  const SAMPLE_DOC = `
+# Visual Settings Verification
+
+## Code Block
+\`\`\`javascript
+function calculateSum(a, b) {
+  return a + b;
+}
+\`\`\`
+
+## Math
+Here is inline math: $E = mc^2$
+
+And block math:
+$$
+\\int_{0}^{\\infty} e^{-x^2} dx = \\frac{\\sqrt{\\pi}}{2}
+$$
+
+## Diagram
+\`\`\`mermaid
+graph TD
+  A[Start] --> B{Decision}
+  B -->|Yes| C[Success]
+  B -->|No| D[Retry]
+\`\`\`
+
+## Emoji
+Greetings: :wave: :thumbsup: :smile:
+`;
+
+  test('panel opens and closes via header button, close button, and Esc key with focus trap', async ({ page }) => {
+    await openApp(page);
+
+    const visualBtn = page.locator('#visual-settings-button');
+    await expect(visualBtn).toBeVisible();
+
+    // Open panel
+    await visualBtn.click();
+    const modal = page.locator('#visual-settings-modal');
+    await expect(modal).toBeVisible();
+    await expect(modal).toHaveAttribute('aria-hidden', 'false');
+
+    // Check first control has focus
+    const hljsSelect = page.locator('#visual-setting-hljs-theme');
+    await expect(hljsSelect).toBeFocused();
+
+    // Verify all 5 controls are present with proper labels
+    await expect(page.locator('#visual-setting-hljs-theme')).toBeVisible();
+    await expect(page.locator('#visual-setting-math-font')).toBeVisible();
+    await expect(page.locator('#visual-setting-mermaid-theme')).toBeVisible();
+    await expect(page.locator('#visual-setting-mermaid-look')).toBeVisible();
+    await expect(page.locator('#visual-setting-emoji-skin-tone')).toBeVisible();
+
+    // Close via Esc key
+    await page.keyboard.press('Escape');
+    await expect(modal).not.toBeVisible();
+    await expect(modal).toHaveAttribute('aria-hidden', 'true');
+
+    // Reopen and close via Close button
+    await visualBtn.click();
+    await expect(modal).toBeVisible();
+    await page.locator('#visual-settings-close-btn').click();
+    await expect(modal).not.toBeVisible();
+
+    // Reopen and close via X icon
+    await visualBtn.click();
+    await expect(modal).toBeVisible();
+    await page.locator('#visual-settings-close-icon').click();
+    await expect(modal).not.toBeVisible();
+
+    // Open via workspace settings dropdown menu
+    await page.locator('#workspaceSettingsDropdown').click();
+    await expect(page.locator('#menu-visual-settings-button')).toBeVisible();
+    await page.locator('#menu-visual-settings-button').click();
+    await expect(modal).toBeVisible();
+    await page.locator('#visual-settings-close-btn').click();
+    await expect(modal).not.toBeVisible();
+  });
+
+  test('all settings update live, re-render preview, persist across reloads, and reset', async ({ page }) => {
+    await openApp(page);
+
+    // Set editor content with code, math, mermaid, and emoji
+    const editor = page.locator('#markdown-editor');
+    await editor.fill(SAMPLE_DOC);
+    await editor.dispatchEvent('input');
+
+    // Open settings panel
+    await page.locator('#visual-settings-button').click();
+    await expect(page.locator('#visual-settings-modal')).toBeVisible();
+
+    // 1. Change highlight.js theme
+    const hljsSelect = page.locator('#visual-setting-hljs-theme');
+    await hljsSelect.selectOption('monokai');
+
+    // 2. Change MathJax font
+    const mathFontSelect = page.locator('#visual-setting-math-font');
+    await mathFontSelect.selectOption('mathjax-stix2');
+
+    // 3. Change Mermaid theme and look
+    const mermaidThemeSelect = page.locator('#visual-setting-mermaid-theme');
+    await mermaidThemeSelect.selectOption('forest');
+
+    const mermaidLookSelect = page.locator('#visual-setting-mermaid-look');
+    await mermaidLookSelect.selectOption('handDrawn');
+
+    // 4. Change JoyPixels skin tone
+    const emojiToneSelect = page.locator('#visual-setting-emoji-skin-tone');
+    await emojiToneSelect.selectOption('tone3');
+
+    // Verify localStorage persistence
+    const storedRaw = await page.evaluate(() => localStorage.getItem('mdv.visualSettings.v1'));
+    expect(storedRaw).toBeTruthy();
+    const stored = JSON.parse(storedRaw);
+    expect(stored.hljsTheme).toBe('monokai');
+    expect(stored.mathFont).toBe('mathjax-stix2');
+    expect(stored.mermaidTheme).toBe('forest');
+    expect(stored.mermaidLook).toBe('handDrawn');
+    expect(stored.emojiSkinTone).toBe('tone3');
+
+    // Close modal
+    await page.locator('#visual-settings-close-btn').click();
+    await expect(page.locator('#visual-settings-modal')).not.toBeVisible();
+
+    // Reload page and check persistence
+    await page.reload();
+    await waitForAppReady(page);
+
+    // Verify settings persisted in store
+    const reloadedSettings = await page.evaluate(() => window.visualSettingsStore.getSettings());
+    expect(reloadedSettings.hljsTheme).toBe('monokai');
+    expect(reloadedSettings.mathFont).toBe('mathjax-stix2');
+    expect(reloadedSettings.mermaidTheme).toBe('forest');
+    expect(reloadedSettings.mermaidLook).toBe('handDrawn');
+    expect(reloadedSettings.emojiSkinTone).toBe('tone3');
+
+    // Verify modal selects reflect persisted settings
+    await page.locator('#visual-settings-button').click();
+    await expect(page.locator('#visual-setting-hljs-theme')).toHaveValue('monokai');
+    await expect(page.locator('#visual-setting-math-font')).toHaveValue('mathjax-stix2');
+    await expect(page.locator('#visual-setting-mermaid-theme')).toHaveValue('forest');
+    await expect(page.locator('#visual-setting-mermaid-look')).toHaveValue('handDrawn');
+    await expect(page.locator('#visual-setting-emoji-skin-tone')).toHaveValue('tone3');
+
+    // Test "Reset to defaults"
+    await page.locator('#visual-settings-reset-btn').click();
+
+    // Verify UI reset
+    await expect(page.locator('#visual-setting-hljs-theme')).toHaveValue('auto');
+    await expect(page.locator('#visual-setting-math-font')).toHaveValue('mathjax-modern');
+    await expect(page.locator('#visual-setting-mermaid-theme')).toHaveValue('auto');
+    await expect(page.locator('#visual-setting-mermaid-look')).toHaveValue('classic');
+    await expect(page.locator('#visual-setting-emoji-skin-tone')).toHaveValue('default');
+
+    // Verify storage reset
+    const resetStored = await page.evaluate(() => JSON.parse(localStorage.getItem('mdv.visualSettings.v1')));
+    expect(resetStored.hljsTheme).toBe('auto');
+    expect(resetStored.mathFont).toBe('mathjax-modern');
+    expect(resetStored.mermaidTheme).toBe('auto');
+    expect(resetStored.mermaidLook).toBe('classic');
+    expect(resetStored.emojiSkinTone).toBe('default');
+
+    await page.locator('#visual-settings-close-btn').click();
+  });
+
+  test('visual settings work in dark mode and mobile viewport', async ({ page }) => {
+    // Set mobile viewport
+    await page.setViewportSize({ width: 375, height: 667 });
+    await openApp(page);
+
+    // Open mobile menu
+    const mobileMenuToggle = page.locator('#mobile-menu-toggle');
+    await mobileMenuToggle.click();
+    await expect(page.locator('#mobile-menu-panel')).toBeVisible();
+
+    // Open settings section inside mobile menu
+    const mobileSettingsToggle = page.locator('[data-mobile-menu-section-toggle][aria-controls="mobile-menu-settings-panel"]');
+    await mobileSettingsToggle.click();
+
+    // Click mobile visual settings button
+    const mobileVisualBtn = page.locator('#mobile-visual-settings-button');
+    await expect(mobileVisualBtn).toBeVisible();
+    await mobileVisualBtn.click();
+
+    // Modal opens on mobile
+    const modal = page.locator('#visual-settings-modal');
+    await expect(modal).toBeVisible();
+
+    // Change setting on mobile
+    await page.locator('#visual-setting-hljs-theme').selectOption('dracula');
+    await page.locator('#visual-settings-close-btn').click();
+    await expect(modal).not.toBeVisible();
+
+    // Switch to dark appearance
+    await page.evaluate(() => {
+      document.documentElement.setAttribute('data-theme', 'dark');
+      const toggle = document.getElementById('theme-toggle');
+      if (toggle) toggle.click();
+    });
+
+    // Reopen and verify dark mode styling
+    await mobileMenuToggle.click();
+    await mobileSettingsToggle.click();
+    await mobileVisualBtn.click();
+    await expect(modal).toBeVisible();
+
+    const selectBg = await page.locator('#visual-setting-hljs-theme').evaluate(el => {
+      return window.getComputedStyle(el).backgroundColor;
+    });
+    expect(selectBg).toBeTruthy();
+
+    await page.locator('#visual-settings-close-btn').click();
+  });
+
+  test('P4 & P5: all 14 highlight.js themes resolve valid stylesheets and dark themes have readable backgrounds in light mode', async ({ page }) => {
+    await openApp(page);
+
+    // Verify all 14 theme options exist
+    const options = await page.evaluate(() => window.visualSettingsAdapters.getOptions().hljsTheme.map(o => o.value));
+    const themeValues = options.filter(v => v !== 'auto');
+    expect(themeValues.length).toBe(14);
+
+    // Test base16 themes (P4)
+    for (const theme of ['dracula', 'solarized-dark', 'solarized-light']) {
+      await page.evaluate(async (t) => {
+        await window.visualSettingsAdapters.applyHighlightTheme(t);
+      }, theme);
+      const linkHref = await page.locator('#mdv-hljs-theme-link').getAttribute('href');
+      expect(linkHref).toContain(`base16/${theme}.min.css`);
+    }
+
+    // Set sample code in light mode (P5)
+    await page.evaluate(() => document.documentElement.setAttribute('data-theme', 'light'));
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('```javascript\nfunction test() { return 42; }\n```');
+    await editor.dispatchEvent('input');
+    await page.waitForTimeout(500);
+
+    // Apply Monokai
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyHighlightTheme('monokai');
+    });
+
+    const preBg = await page.locator('#markdown-preview pre').first().evaluate(el => {
+      return window.getComputedStyle(el).backgroundColor;
+    });
+    const codeColor = await page.locator('#markdown-preview pre code').first().evaluate(el => {
+      return window.getComputedStyle(el).color;
+    });
+
+    // Dark theme should NOT be transparent in light mode
+    expect(preBg).not.toBe('rgba(0, 0, 0, 0)');
+    expect(preBg).not.toBe('transparent');
+    expect(preBg).toContain('39, 40, 34'); // #272822
+    expect(codeColor).toContain('221, 221, 221'); // #ddd
+    const fnColor = await page.locator('#markdown-preview pre code .hljs-title').first().evaluate(el => {
+      return window.getComputedStyle(el).color;
+    });
+    expect(fnColor).toBe('rgb(166, 226, 46)'); // Monokai green
+  });
+
+  test('P1: MathJax font switches to STIX Two and updates output jax font data live and on reload', async ({ page }) => {
+    await openApp(page, '/', { mockMathJax: false });
+
+    // Fill math content
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('$$E = mc^2$$');
+    await editor.dispatchEvent('input');
+    await page.locator('#markdown-preview mjx-container').first().waitFor({ timeout: 15_000 });
+
+    // Apply STIX Two font live
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyMathFont('mathjax-stix2');
+    });
+
+    const fontState = await page.evaluate(() => {
+      const mj = window.MathJax;
+      const font = mj?.startup?.output?.font;
+      const fontPkg = mj?._?.output?.fonts?.[window.visualSettingsAdapters?.getCurrentMathFont()];
+      const fontClassKey = fontPkg?.chtml_ts ? Object.keys(fontPkg.chtml_ts)[0] : null;
+      return {
+        activeFont: window.visualSettingsAdapters?.getCurrentMathFont(),
+        hasFontData: !!mj?.config?.chtml?.fontData,
+        fontClassKey,
+        cssFontPrefix: font?.cssFontPrefix
+      };
+    });
+
+    expect(fontState.activeFont).toBe('mathjax-stix2');
+    expect(fontState.hasFontData).toBe(true);
+    expect(fontState.fontClassKey).toBe('MathJaxStix2Font');
+    expect(fontState.cssFontPrefix).toBe('STX');
+  });
+
+  test('P6 & P7: split preview refreshes emoji, math, mermaid, and syntax themes across both panes', async ({ page }) => {
+    await openApp(page, '/', { mockMathJax: false });
+
+    // Primary document with emoji, math formula, mermaid diagram, and code block
+    const primaryContent = '# Primary\n\n:wave:\n\n$$\\sum_{i=1}^n i$$\n\n```mermaid\ngraph TD\n  A-->B\n```\n\n```javascript\nconst a = 1;\n```';
+    const secondaryContent = '# Secondary\n\n:wave:\n\n$$\\int_0^\\infty x dx$$\n\n```mermaid\ngraph TD\n  C-->D\n```\n\n```javascript\nconst b = 2;\n```';
+
+    const editor = page.locator('#markdown-editor');
+    await editor.fill(primaryContent);
+    await editor.dispatchEvent('input');
+    await page.waitForTimeout(1000);
+
+    // Create a second tab for split view test (P6)
+    await page.locator('#tab-new-btn').click();
+    await editor.fill(secondaryContent);
+    await editor.dispatchEvent('input');
+    await page.waitForTimeout(500);
+
+    // Open first tab in split view
+    const secondTabId = await page.locator('#tab-list .tab-item.active').getAttribute('data-tab-id');
+    await page.locator('#tab-list .tab-item').first().click({ button: 'right' });
+    await page.getByRole('menuitem', { name: 'Open in split view' }).click();
+    await page.locator('#document-split-destination').selectOption(secondTabId);
+    await page.locator('#document-split-modal-confirm').click();
+
+    // Switch to preview mode to see split preview
+    await page.locator('button.view-toggle-btn[data-view-mode="preview"]').click();
+
+    // Verify initial yellow wave in both panes
+    await expect(page.locator('#markdown-preview')).toContainText('👋');
+    await expect(page.locator('#document-split-preview')).toContainText('👋');
+
+    // Verify math, mermaid, and code blocks rendered in both panes
+    await expect(page.locator('#markdown-preview mjx-container')).toBeVisible({ timeout: 15_000 });
+    await expect(page.locator('#document-split-preview mjx-container')).toBeVisible({ timeout: 15_000 });
+    await expect(page.locator('#markdown-preview .mermaid svg')).toBeVisible({ timeout: 15_000 });
+    await expect(page.locator('#document-split-preview .mermaid svg')).toBeVisible({ timeout: 15_000 });
+    await expect(page.locator('#markdown-preview pre code.hljs')).toBeVisible();
+    await expect(page.locator('#document-split-preview pre code.hljs')).toBeVisible();
+
+    // 1. Change emoji skin tone to tone3
+    await page.locator('#visual-settings-button').click();
+    await page.locator('#visual-setting-emoji-skin-tone').selectOption('tone3');
+    await page.locator('#visual-settings-close-btn').click();
+
+    // Verify both primary and split preview refresh to tone3 (P6 & P7)
+    await expect.poll(async () => {
+      const primary = await page.locator('#markdown-preview').textContent();
+      const secondary = await page.locator('#document-split-preview').textContent();
+      return { primary: primary.includes('👋🏽'), secondary: secondary.includes('👋🏽') };
+    }, { timeout: 10_000 }).toEqual({ primary: true, secondary: true });
+
+    // 2. Change hljs theme to dracula (Base16 theme)
+    await page.locator('#visual-settings-button').click();
+    await page.locator('#visual-setting-hljs-theme').selectOption('dracula');
+    await page.locator('#visual-settings-close-btn').click();
+
+    await expect.poll(async () => {
+      return page.evaluate(() => document.getElementById('mdv-hljs-theme-link')?.href || '');
+    }, { timeout: 10_000 }).toContain('styles/base16/dracula.min.css');
+
+    // 3. Change math font to STIX Two
+    await page.locator('#visual-settings-button').click();
+    await page.locator('#visual-setting-math-font').selectOption('mathjax-stix2');
+    await page.locator('#visual-settings-close-btn').click();
+
+    await expect.poll(async () => {
+      return page.evaluate(() => {
+        const adapters = window.visualSettingsAdapters;
+        return adapters ? adapters.getCurrentMathFont() : null;
+      });
+    }, { timeout: 15_000 }).toBe('mathjax-stix2');
+
+    // 4. Change mermaid theme to dark and look to handDrawn
+    await page.locator('#visual-settings-button').click();
+    await page.locator('#visual-setting-mermaid-theme').selectOption('dark');
+    await page.locator('#visual-setting-mermaid-look').selectOption('handDrawn');
+    await page.locator('#visual-settings-close-btn').click();
+
+    await expect.poll(async () => {
+      return page.evaluate(() => window.visualSettingsAdapters?.getMermaidConfig());
+    }, { timeout: 10_000 }).toMatchObject({ theme: 'dark', look: 'handDrawn' });
+
+    // 5. Verify reload persistence
+    await page.reload();
+    await page.waitForLoadState('domcontentloaded');
+
+    const persisted = await page.evaluate(() => window.visualSettingsStore.getSettings());
+    expect(persisted.emojiSkinTone).toBe('tone3');
+    expect(persisted.hljsTheme).toBe('dracula');
+    expect(persisted.mathFont).toBe('mathjax-stix2');
+    expect(persisted.mermaidTheme).toBe('dark');
+    expect(persisted.mermaidLook).toBe('handDrawn');
+  });
+
+  test('P3: Dockerfile includes visual settings scripts and distribution assets are complete', async ({ page }) => {
+    const fs = require('fs');
+    const path = require('path');
+    const dockerfile = fs.readFileSync(path.resolve(__dirname, '../../Dockerfile'), 'utf8');
+    expect(dockerfile).toContain('COPY visual-settings-adapters.js');
+    expect(dockerfile).toContain('COPY visual-settings-store.js');
+
+    const sw = fs.readFileSync(path.resolve(__dirname, '../../sw.js'), 'utf8');
+    expect(sw).toContain('visual-settings-adapters.js');
+    expect(sw).toContain('visual-settings-store.js');
+  });
+
+  test('P1: MathJax configuration is compatible with desktop MathJax bundle and legacy bundles', async ({ page }) => {
+    // 1. Verify desktop prepare.js downloads MathJax 4 bundle with SRI hash
+    const fs = require('fs');
+    const path = require('path');
+    const prepareSource = fs.readFileSync(path.resolve(__dirname, '../../desktop-app/prepare.js'), 'utf8');
+    expect(prepareSource).toContain('https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7/tex-mml-chtml.js');
+    expect(prepareSource).toContain('sha384-2f5bAKjuFIbbQ+0O9aSu5W1OyLA4q80QrDEvjXDwJexjU9VqoJhMgw19pYVpSJab');
+
+    // 2. Verify configureMathJax does not pass font name string to chtml.font
+    const testPage = await page.context().newPage();
+    await testPage.addInitScript(() => {
+      window.__initialChtmlFont = null;
+      let _mj = undefined;
+      Object.defineProperty(window, 'MathJax', {
+        configurable: true,
+        enumerable: true,
+        get: () => _mj,
+        set: (v) => {
+          _mj = v;
+          if (v && v.chtml) {
+            window.__initialChtmlFont = v.chtml.font;
+          }
+        }
+      });
+    });
+
+    await openApp(testPage, '/', { mockMathJax: false });
+    const initialChtmlFont = await testPage.evaluate(() => window.__initialChtmlFont);
+    expect(typeof initialChtmlFont).not.toBe('string');
+    await testPage.close();
+
+    // 3. Verify MathJax 3 bundle dependency is compatible with configuration without crashing startup
+    const legacyPage = await page.context().newPage();
+    const errors = [];
+    legacyPage.on('pageerror', err => errors.push(err.message));
+    legacyPage.on('console', msg => {
+      if (msg.type() === 'error') errors.push(msg.text());
+    });
+    await legacyPage.setContent(`<!DOCTYPE html><html><head>
+      <script>
+        window.MathJax = {
+          startup: { typeset: false },
+          options: { a11y: { inTabOrder: false } },
+          output: { font: 'mathjax-modern' },
+          tex: {
+            inlineMath: [['$', '$'], ['\\\\(', '\\\\)']],
+            displayMath: [['$$', '$$'], ['\\\\[', '\\\\]']],
+            processEscapes: true
+          }
+        };
+      </script>
+      <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.min.js"></script>
+    </head><body><div id="math">$$x^2$$</div></body></html>`);
+    await legacyPage.waitForTimeout(1500);
+
+    const legacyResult = await legacyPage.evaluate(async () => {
+      if (window.MathJax?.startup?.promise) {
+        await window.MathJax.startup.promise;
+      }
+      let rendered = false;
+      if (typeof window.MathJax?.typesetPromise === 'function') {
+        await window.MathJax.typesetPromise([document.getElementById('math')]);
+        rendered = !!document.querySelector('mjx-container');
+      }
+      return {
+        hasTypesetPromise: typeof window.MathJax?.typesetPromise === 'function',
+        rendered
+      };
+    });
+    expect(errors.filter(e => e.includes('adaptiveCSS')).length).toBe(0);
+    expect(legacyResult.hasTypesetPromise).toBe(true);
+    expect(legacyResult.rendered).toBe(true);
+    await legacyPage.close();
+  });
+
+  test('P2: MathJax font switches font resource URLs and dynamic font packages without failed glyph loads', async ({ page }) => {
+    const failedFontRequests = [];
+    page.on('response', resp => {
+      if (resp.status() >= 400 && (resp.url().includes('woff') || resp.url().includes('dynamic') || resp.url().includes('font'))) {
+        failedFontRequests.push({ url: resp.url(), status: resp.status() });
+      }
+    });
+
+    const wrongPackageRequests = [];
+    page.on('request', req => {
+      if (req.url().includes('mathjax-modern-font') && (req.url().includes('stx') || req.url().includes('asn'))) {
+        wrongPackageRequests.push(req.url());
+      }
+    });
+
+    await openApp(page, '/', { mockMathJax: false });
+
+    // Initial render with Modern
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('$$x^2$$\n\n$$\\mathbb{R}$$');
+    await editor.dispatchEvent('input');
+    await expect(page.locator('#markdown-preview mjx-container').first()).toBeVisible({ timeout: 15_000 });
+
+    // 1. Switch to STIX Two live via adapter
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyMathFont('mathjax-stix2');
+    });
+
+    // Check font resource options on startup.output.font
+    const stixFontOptions = await page.evaluate(() => {
+      const font = window.MathJax?.startup?.output?.font;
+      return {
+        prefix: font?.cssFontPrefix,
+        fontURL: font?.options?.fontURL,
+        dynamicPrefix: font?.options?.dynamicPrefix
+      };
+    });
+    expect(stixFontOptions.prefix).toBe('STX');
+    expect(stixFontOptions.fontURL).toContain('mathjax-stix2-font/chtml/woff');
+    expect(stixFontOptions.dynamicPrefix).toBe('[mathjax-stix2]/chtml/dynamic');
+
+    // 2. Switch to Asana live with \mathbb{R}
+    let asanaDynamicLoaded = false;
+    page.on('request', req => {
+      if (req.url().includes('mathjax-asana-font') && req.url().includes('double-struck.js')) {
+        asanaDynamicLoaded = true;
+      }
+    });
+
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyMathFont('mathjax-asana');
+    });
+
+    const asanaFontOptions = await page.evaluate(() => {
+      const font = window.MathJax?.startup?.output?.font;
+      return {
+        prefix: font?.cssFontPrefix,
+        fontURL: font?.options?.fontURL,
+        dynamicPrefix: font?.options?.dynamicPrefix
+      };
+    });
+    expect(asanaFontOptions.prefix).toBe('ASNA');
+    expect(asanaFontOptions.fontURL).toContain('mathjax-asana-font/chtml/woff');
+    expect(asanaFontOptions.dynamicPrefix).toBe('[mathjax-asana]/chtml/dynamic');
+
+    // Verify equations are rendered and no failed font requests occurred
+    await expect(page.locator('#markdown-preview mjx-container')).toHaveCount(2);
+    expect(failedFontRequests).toEqual([]);
+    expect(wrongPackageRequests).toEqual([]);
+  });
+
+  test('P2: load Modern when resetting after an alternate-font reload', async ({ page }) => {
+    await page.addInitScript(() => {
+      localStorage.setItem('mdv.visualSettings.v1', JSON.stringify({
+        mathFont: 'mathjax-stix2'
+      }));
+    });
+
+    const failedFontRequests = [];
+    page.on('response', resp => {
+      if (resp.url().includes('mathjax') && resp.status() >= 400) {
+        failedFontRequests.push({ url: resp.url(), status: resp.status() });
+      }
+    });
+
+    await openApp(page, '/', { mockMathJax: false });
+
+    // Initial render with STIX Two
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('$$x^2 + y^2 = z^2$$');
+    await editor.dispatchEvent('input');
+    await expect(page.locator('#markdown-preview mjx-container')).toBeVisible({ timeout: 15_000 });
+
+    const initialBootPrefix = await page.evaluate(() => {
+      return window.MathJax?.startup?.output?.font?.cssFontPrefix;
+    });
+    expect(initialBootPrefix).toBe('STX');
+
+    // Reset visual settings to default (Modern math font)
+    await page.locator('#visual-settings-button').click();
+    await page.locator('#visual-settings-reset-btn').click();
+
+    await expect(page.locator('#visual-setting-math-font')).toHaveValue('mathjax-modern');
+
+    await expect.poll(async () => {
+      return page.evaluate(() => {
+        const font = window.MathJax?.startup?.output?.font;
+        return {
+          prefix: font?.cssFontPrefix,
+          fontURL: font?.options?.fontURL,
+          dynamicPrefix: font?.options?.dynamicPrefix,
+          activeFont: window.visualSettingsAdapters?.getCurrentMathFont()
+        };
+      });
+    }, { timeout: 15_000 }).toEqual({
+      prefix: 'MM',
+      fontURL: 'https://cdn.jsdelivr.net/npm/mathjax-modern-font/chtml/woff',
+      dynamicPrefix: '[mathjax-modern]/chtml/dynamic',
+      activeFont: 'mathjax-modern'
+    });
+
+    await expect(page.locator('#markdown-preview mjx-container')).toBeVisible();
+    expect(failedFontRequests).toEqual([]);
+  });
+
+  test('P2: resolve automatic code colors against PDF export theme', async ({ page }) => {
+    await openApp(page);
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('```javascript\nfunction greet() { return "hello"; }\n```');
+    await editor.dispatchEvent('input');
+    await expect(page.locator('#markdown-preview pre code.hljs')).toBeVisible({ timeout: 15_000 });
+
+    // Ensure Auto syntax highlighting is active
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyHighlightTheme('auto');
+    });
+
+    // 1. Light app theme + Auto syntax + Dark PDF export
+    await page.evaluate(() => {
+      document.documentElement.setAttribute('data-theme', 'light');
+      window.visualSettingsAdapters.onAppThemeChange('light');
+    });
+
+    // Preview code block has light background (app theme)
+    const previewLightBg = await page.locator('#markdown-preview pre').first().evaluate(el => {
+      return window.getComputedStyle(el).backgroundColor;
+    });
+    expect(previewLightBg).not.toBe('rgb(22, 27, 34)');
+
+    // Trigger dark vector PDF export to inspect the export render snapshot
+    await page.evaluate(() => {
+      window.print = () => { window.__darkPdfPrintReady = true; };
+    });
+    await page.locator('#export-pdf').dispatchEvent('click');
+    await page.locator('#pdf-export-theme-card-dark').click();
+    await page.locator('#pdf-export-confirm').click();
+
+    await expect.poll(() => page.evaluate(() => window.__darkPdfPrintReady), { timeout: 20_000 }).toBe(true);
+
+    // Evaluate snapshot code block colors under dark export
+    const darkExportColors = await page.locator('.browser-print-export-snapshot').evaluate(root => {
+      const pre = root.querySelector('pre');
+      const code = root.querySelector('code');
+      return {
+        preBg: window.getComputedStyle(pre).backgroundColor,
+        codeColor: window.getComputedStyle(code).color
+      };
+    });
+
+    expect(darkExportColors.preBg).toBe('rgb(22, 27, 34)'); // #161b22
+    expect(darkExportColors.codeColor).toBe('rgb(201, 209, 217)'); // #c9d1d9
+
+    // Close/restore afterprint
+    await page.evaluate(() => {
+      window.dispatchEvent(new Event('afterprint'));
+    });
+
+    // 2. Dark app theme + Auto syntax + Light PDF export
+    await page.evaluate(() => {
+      document.documentElement.setAttribute('data-theme', 'dark');
+      window.visualSettingsAdapters.onAppThemeChange('dark');
+      window.print = () => { window.__lightPdfPrintReady = true; };
+    });
+
+    // Preview code block has dark background in dark mode
+    const previewDarkBg = await page.locator('#markdown-preview pre').first().evaluate(el => {
+      return window.getComputedStyle(el).backgroundColor;
+    });
+    expect(previewDarkBg).not.toBe('rgb(246, 248, 250)');
+
+    await page.locator('#export-pdf').dispatchEvent('click');
+    await page.locator('#pdf-export-theme-card-light').click();
+    await page.locator('#pdf-export-confirm').click();
+
+    await expect.poll(() => page.evaluate(() => window.__lightPdfPrintReady), { timeout: 20_000 }).toBe(true);
+
+    const lightExportColors = await page.locator('.browser-print-export-snapshot').evaluate(root => {
+      const pre = root.querySelector('pre');
+      const code = root.querySelector('code');
+      return {
+        preBg: window.getComputedStyle(pre).backgroundColor,
+        codeColor: window.getComputedStyle(code).color
+      };
+    });
+
+    expect(lightExportColors.preBg).toBe('rgb(246, 248, 250)'); // #f6f8fa
+    expect(lightExportColors.codeColor).toBe('rgb(36, 41, 46)'); // #24292e
+
+    await page.evaluate(() => {
+      window.dispatchEvent(new Event('afterprint'));
+    });
+  });
+
+  test('P2: remove built-in token palette when applying syntax theme and match external stylesheets', async ({ page }) => {
+    await openApp(page);
+    const editor = page.locator('#markdown-editor');
+    await editor.fill('```javascript\nfunction hello() { return 42; }\n```');
+    await editor.dispatchEvent('input');
+    await expect(page.locator('#markdown-preview pre code.hljs')).toBeVisible({ timeout: 15_000 });
+
+    // 1. In Auto mode (light app), function title uses the built-in GitHub purple
+    await page.evaluate(async () => {
+      document.documentElement.setAttribute('data-theme', 'light');
+      await window.visualSettingsAdapters.applyHighlightTheme('auto');
+    });
+    const autoFnColor = await page.locator('#markdown-preview pre code .hljs-title').first().evaluate(el => {
+      return window.getComputedStyle(el).color;
+    });
+    expect(autoFnColor).toBe('rgb(111, 66, 193)');
+
+    // 2. Select Monokai: function name must be Monokai green, NOT legacy purple
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyHighlightTheme('monokai');
+    });
+    await expect.poll(async () => {
+      return await page.locator('#markdown-preview pre code .hljs-title').first().evaluate(el => {
+        return window.getComputedStyle(el).color;
+      });
+    }, { timeout: 10_000 }).toBe('rgb(166, 226, 46)');
+
+    // 3. Compare actual token colors against an isolated iframe containing only the selected stylesheet
+    const testThemes = ['monokai', 'dracula', 'atom-one-dark', 'solarized-dark', 'nord', 'default'];
+    for (const theme of testThemes) {
+      await page.evaluate(async (t) => {
+        await window.visualSettingsAdapters.applyHighlightTheme(t);
+      }, theme);
+
+      const comparison = await page.evaluate(async () => {
+        const link = document.getElementById('mdv-hljs-theme-link');
+        const href = link ? link.href : '';
+
+        const iframe = document.createElement('iframe');
+        iframe.style.position = 'fixed';
+        iframe.style.top = '-9999px';
+        iframe.style.left = '-9999px';
+        document.body.appendChild(iframe);
+
+        const idoc = iframe.contentDocument;
+        idoc.open();
+        idoc.write(`<!DOCTYPE html><html><head><link rel="stylesheet" href="${href}"></head><body><pre><code class="hljs language-javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">hello</span>() { <span class="hljs-keyword">return</span> <span class="hljs-number">42</span>; }</code></pre></body></html>`);
+        idoc.close();
+
+        await new Promise(resolve => {
+          const l = idoc.querySelector('link');
+          if (l && l.sheet) return resolve();
+          if (l) {
+            l.onload = () => resolve();
+            l.onerror = () => resolve();
+          }
+          setTimeout(resolve, 1000);
+        });
+
+        const previewFn = document.querySelector('#markdown-preview pre code .hljs-title');
+        const iframeFn = idoc.querySelector('pre code .hljs-title');
+        const previewKw = document.querySelector('#markdown-preview pre code .hljs-keyword');
+        const iframeKw = idoc.querySelector('pre code .hljs-keyword');
+
+        const result = {
+          previewFnColor: previewFn ? getComputedStyle(previewFn).color : null,
+          iframeFnColor: iframeFn ? iframe.contentWindow.getComputedStyle(iframeFn).color : null,
+          previewKwColor: previewKw ? getComputedStyle(previewKw).color : null,
+          iframeKwColor: iframeKw ? iframe.contentWindow.getComputedStyle(iframeKw).color : null
+        };
+        iframe.remove();
+        return result;
+      });
+
+      expect(comparison.previewFnColor).toBe(comparison.iframeFnColor);
+      expect(comparison.previewKwColor).toBe(comparison.iframeKwColor);
+    }
+
+    // 4. Switching back to Auto restores built-in palette
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyHighlightTheme('auto');
+    });
+    await expect.poll(async () => {
+      return await page.locator('#markdown-preview pre code .hljs-title').first().evaluate(el => {
+        return window.getComputedStyle(el).color;
+      });
+    }, { timeout: 10_000 }).toBe('rgb(111, 66, 193)');
+  });
+
+  test('P2: canonicalize emoji aliases before adding skin-tone suffix', async ({ page }) => {
+    await openApp(page);
+    const editor = page.locator('#markdown-editor');
+    await editor.fill(':thumbsup: :+1: :runner: :wave_tone3: :smile:');
+    await editor.dispatchEvent('input');
+    await expect(page.locator('#markdown-preview')).toContainText('👋🏽', { timeout: 15_000 });
+
+    const expectedGlyphs = {
+      tone1: { thumbs: '👍🏻', runner: '🏃🏻' },
+      tone2: { thumbs: '👍🏼', runner: '🏃🏼' },
+      tone3: { thumbs: '👍🏽', runner: '🏃🏽' },
+      tone4: { thumbs: '👍🏾', runner: '🏃🏾' },
+      tone5: { thumbs: '👍🏿', runner: '🏃🏿' }
+    };
+
+    // Test across all 5 non-default skin tones in light and dark mode
+    for (const [tone, glyphs] of Object.entries(expectedGlyphs)) {
+      for (const appTheme of ['light', 'dark']) {
+        await page.evaluate(async ({ tone, appTheme }) => {
+          document.documentElement.setAttribute('data-theme', appTheme);
+          await window.visualSettingsAdapters.applyEmojiSkinTone(tone);
+        }, { tone, appTheme });
+
+        await expect.poll(async () => {
+          return await page.locator('#markdown-preview').textContent();
+        }, { timeout: 10_000 }).toContain(glyphs.thumbs);
+
+        const previewText = await page.locator('#markdown-preview').textContent();
+        // Both canonical :thumbsup: and alias :+1: must render with the chosen skin tone
+        const thumbsOccurrences = (previewText.match(new RegExp(glyphs.thumbs, 'g')) || []).length;
+        expect(thumbsOccurrences).toBe(2);
+
+        // Alias :runner: must render with the chosen skin tone (not yellow 🏃)
+        expect(previewText).toContain(glyphs.runner);
+
+        // Explicit tone :wave_tone3: preserved
+        expect(previewText).toContain('👋🏽');
+
+        // Genuinely unsupported emoji :smile: remains 😄
+        expect(previewText).toContain('😄');
+      }
+    }
+
+    // Reset to default
+    await page.evaluate(async () => {
+      await window.visualSettingsAdapters.applyEmojiSkinTone('default');
+    });
+    await expect.poll(async () => {
+      return await page.locator('#markdown-preview').textContent();
+    }, { timeout: 10_000 }).toContain('👍');
+    const defaultText = await page.locator('#markdown-preview').textContent();
+    const defaultThumbs = (defaultText.match(/👍/g) || []).length;
+    expect(defaultThumbs).toBe(2);
+    expect(defaultText).toContain('🏃');
+    expect(defaultText).toContain('👋🏽');
+  });
+});
+
+
diff --git a/tests/helpers/app.js b/tests/helpers/app.js
index f31a45d9..9c5eecf9 100644
--- a/tests/helpers/app.js
+++ b/tests/helpers/app.js
@@ -33,7 +33,7 @@ async function appVersion() {
   return appVersionPromise;
 }
 
-async function installLocalBrowserAssets(page) {
+async function installLocalBrowserAssets(page, options = {}) {
   if (routedPages.has(page)) return;
   routedPages.add(page);
 
@@ -50,26 +50,28 @@ async function installLocalBrowserAssets(page) {
     });
   }
 
-  await page.addInitScript(() => {
-    window.MathJax = {
-      startup: { promise: Promise.resolve() },
-      typesetPromise(targets = []) {
-        targets.forEach(target => {
-          if (target.querySelector('mjx-container[data-test-renderer]')) return;
-          const rendered = document.createElement('mjx-container');
-          rendered.dataset.testRenderer = 'true';
-          rendered.textContent = 'Rendered math';
-          target.appendChild(rendered);
-        });
-        return Promise.resolve();
-      },
-      typesetClear(targets = []) {
-        targets.forEach(target => {
-          target.querySelectorAll('mjx-container[data-test-renderer]').forEach(node => node.remove());
-        });
-      }
-    };
-  });
+  if (options.mockMathJax !== false) {
+    await page.addInitScript(() => {
+      window.MathJax = {
+        startup: { promise: Promise.resolve() },
+        typesetPromise(targets = []) {
+          targets.forEach(target => {
+            if (target.querySelector('mjx-container[data-test-renderer]')) return;
+            const rendered = document.createElement('mjx-container');
+            rendered.dataset.testRenderer = 'true';
+            rendered.textContent = 'Rendered math';
+            target.appendChild(rendered);
+          });
+          return Promise.resolve();
+        },
+        typesetClear(targets = []) {
+          targets.forEach(target => {
+            target.querySelectorAll('mjx-container[data-test-renderer]').forEach(node => node.remove());
+          });
+        }
+      };
+    });
+  }
 }
 
 async function waitForAppReady(page) {
@@ -91,7 +93,7 @@ function isCriticalBrowserMessage(message) {
 async function openApp(page, url = '/', options = {}) {
   const expectEditorVisible = options.expectEditorVisible !== false;
   const criticalErrors = [];
-  await installLocalBrowserAssets(page);
+  await installLocalBrowserAssets(page, options);
   if (options.showReleaseNotes !== true) {
     const version = await appVersion();
     await page.addInitScript(currentVersion => {
diff --git a/tests/helpers/static-build-check.mjs b/tests/helpers/static-build-check.mjs
index b27fdc15..3e0132d4 100644
--- a/tests/helpers/static-build-check.mjs
+++ b/tests/helpers/static-build-check.mjs
@@ -14,6 +14,8 @@ const requiredFiles = [
   'index.html',
   '404.html',
   'workspace-storage.js',
+  'visual-settings-adapters.js',
+  'visual-settings-store.js',
   'script.js',
   'preview-worker.js',
   'styles.css',
@@ -49,12 +51,16 @@ const syntaxCheckedFiles = [
   'workspace-storage.js',
   'preview-worker.js',
   'sw.js',
+  ...(fs.existsSync(path.join(rootDir, 'visual-settings-store.js')) ? ['visual-settings-store.js'] : []),
+  ...(fs.existsSync(path.join(rootDir, 'visual-settings-adapters.js')) ? ['visual-settings-adapters.js'] : []),
   'assets/seo-metadata.mjs',
   'seo/welcome-content.mjs',
   'desktop-app/resources/js/main.js',
   'desktop-app/resources/js/script.js',
   'desktop-app/resources/js/workspace-storage.js',
   'desktop-app/resources/js/preview-worker.js',
+  ...(fs.existsSync(path.join(rootDir, 'desktop-app/resources/js/visual-settings-store.js')) ? ['desktop-app/resources/js/visual-settings-store.js'] : []),
+  ...(fs.existsSync(path.join(rootDir, 'desktop-app/resources/js/visual-settings-adapters.js')) ? ['desktop-app/resources/js/visual-settings-adapters.js'] : []),
   'assets/i18n/generate-ui-locales.mjs',
   'assets/i18n/audit-ui-locales.mjs'
 ];
@@ -66,12 +72,20 @@ for (const relativePath of syntaxCheckedFiles) {
   });
 }
 
-for (const [sourcePath, desktopPath] of [
+const syncedDesktopFiles = [
   ['script.js', 'desktop-app/resources/js/script.js'],
   ['workspace-storage.js', 'desktop-app/resources/js/workspace-storage.js'],
   ['preview-worker.js', 'desktop-app/resources/js/preview-worker.js'],
   ['styles.css', 'desktop-app/resources/styles.css']
-]) {
+];
+if (fs.existsSync(path.join(rootDir, 'visual-settings-store.js'))) {
+  syncedDesktopFiles.push(['visual-settings-store.js', 'desktop-app/resources/js/visual-settings-store.js']);
+}
+if (fs.existsSync(path.join(rootDir, 'visual-settings-adapters.js'))) {
+  syncedDesktopFiles.push(['visual-settings-adapters.js', 'desktop-app/resources/js/visual-settings-adapters.js']);
+}
+
+for (const [sourcePath, desktopPath] of syncedDesktopFiles) {
   const source = fs.readFileSync(path.join(rootDir, sourcePath));
   const desktopCopy = fs.readFileSync(path.join(rootDir, desktopPath));
   if (!source.equals(desktopCopy)) {
@@ -111,7 +125,7 @@ if (SEO_LOCALES.length !== 15) {
   throw new Error(`Expected 15 supported SEO locales, found ${SEO_LOCALES.length}.`);
 }
 
-if (sitemapXml !== buildSitemap()) {
+if (sitemapXml.replace(/\r\n/g, '\n') !== buildSitemap().replace(/\r\n/g, '\n')) {
   throw new Error('sitemap.xml does not match the canonical locale configuration. Run npm run seo:sitemap.');
 }
 
diff --git a/tests/unit/visual-settings-adapters.test.mjs b/tests/unit/visual-settings-adapters.test.mjs
new file mode 100644
index 00000000..3ce2471e
--- /dev/null
+++ b/tests/unit/visual-settings-adapters.test.mjs
@@ -0,0 +1,427 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import vm from 'node:vm';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const rootDir = path.resolve(__dirname, '../..');
+
+function createAdapterContext() {
+  const createdElements = [];
+
+  const documentElement = {
+    getAttribute(name) {
+      return this._attrs?.[name] || null;
+    },
+    setAttribute(name, val) {
+      this._attrs = this._attrs || {};
+      this._attrs[name] = val;
+    },
+    removeAttribute(name) {
+      if (this._attrs) delete this._attrs[name];
+    }
+  };
+
+  const head = {
+    appendChild(el) {
+      createdElements.push(el);
+      return el;
+    }
+  };
+
+  const document = {
+    documentElement,
+    head,
+    createElement(tag) {
+      let _href = '';
+      let _src = '';
+      const el = {
+        tagName: tag.toUpperCase(),
+        id: '',
+        rel: '',
+        textContent: '',
+        onload: null,
+        onerror: null,
+        get href() { return _href; },
+        set href(val) {
+          _href = val;
+          setTimeout(() => { if (this.onload) this.onload(); }, 1);
+        },
+        get src() { return _src; },
+        set src(val) {
+          _src = val;
+          setTimeout(() => { if (this.onload) this.onload(); }, 1);
+        },
+        setAttribute(k, v) { this[k] = v; },
+        getAttribute(k) { return this[k] || null; },
+        removeAttribute(k) { delete this[k]; }
+      };
+      return el;
+    },
+    getElementById(id) {
+      return createdElements.find(e => e.id === id) || null;
+    },
+    querySelector(selector) {
+      return null;
+    },
+    querySelectorAll(selector) {
+      return [];
+    }
+  };
+
+  class MockMutationObserver {
+    constructor(cb) { this.cb = cb; }
+    observe() {}
+    disconnect() {}
+  }
+
+  const context = {
+    window: {},
+    document,
+    MutationObserver: MockMutationObserver,
+    console: { warn: () => {}, log: () => {} },
+    setTimeout,
+    clearTimeout
+  };
+  context.window = context;
+
+  const code = fs.readFileSync(path.join(rootDir, 'visual-settings-adapters.js'), 'utf8');
+  vm.runInNewContext(code, context);
+
+  return { context, adapters: context.window.visualSettingsAdapters };
+}
+
+test('getOptions() returns valid metadata for all visual settings', () => {
+  const { adapters } = createAdapterContext();
+  const options = adapters.getOptions();
+
+  assert.ok(options.hljsTheme && Array.isArray(options.hljsTheme));
+  assert.ok(options.mathFont && Array.isArray(options.mathFont));
+  assert.ok(options.mermaidTheme && Array.isArray(options.mermaidTheme));
+  assert.ok(options.mermaidLook && Array.isArray(options.mermaidLook));
+  assert.ok(options.emojiSkinTone && Array.isArray(options.emojiSkinTone));
+
+  // Check required values exist
+  const hljsValues = options.hljsTheme.map(o => o.value);
+  assert.ok(hljsValues.includes('auto'));
+  assert.ok(hljsValues.includes('github'));
+  assert.ok(hljsValues.includes('monokai'));
+  assert.ok(hljsValues.includes('dracula'));
+
+  const mathValues = options.mathFont.map(o => o.value);
+  assert.ok(mathValues.includes('mathjax-modern'));
+  assert.ok(mathValues.includes('mathjax-stix2'));
+
+  const mermaidThemes = options.mermaidTheme.map(o => o.value);
+  assert.ok(mermaidThemes.includes('auto'));
+  assert.ok(mermaidThemes.includes('forest'));
+  assert.ok(mermaidThemes.includes('dark'));
+
+  const mermaidLooks = options.mermaidLook.map(o => o.value);
+  assert.ok(mermaidLooks.includes('classic'));
+  assert.ok(mermaidLooks.includes('handDrawn'));
+
+  const tones = options.emojiSkinTone.map(o => o.value);
+  assert.ok(tones.includes('default'));
+  assert.ok(tones.includes('tone1'));
+  assert.ok(tones.includes('tone5'));
+});
+
+test('getDefaults() returns standard default visual settings', () => {
+  const { adapters } = createAdapterContext();
+  const defaults = JSON.parse(JSON.stringify(adapters.getDefaults()));
+
+  assert.deepEqual(defaults, {
+    hljsTheme: 'auto',
+    mathFont: 'mathjax-modern',
+    mermaidTheme: 'auto',
+    mermaidLook: 'classic',
+    emojiSkinTone: 'default'
+  });
+});
+
+test('resolveEmojiShortcode handles skin tones accurately', () => {
+  const { context, adapters } = createAdapterContext();
+
+  // Mock joypixels emojiList and altShortNames
+  context.joypixels = {
+    emojiList: {
+      ':wave:': {},
+      ':wave_tone1:': {},
+      ':wave_tone2:': {},
+      ':wave_tone3:': {},
+      ':wave_tone4:': {},
+      ':wave_tone5:': {},
+      ':thumbsup:': { shortnames: [':+1:', ':thumbup:'] },
+      ':thumbsup_tone1:': { shortnames: [':+1_tone1:'] },
+      ':thumbsup_tone5:': { shortnames: [':+1_tone5:'] },
+      ':person_running:': { shortnames: [':runner:'] },
+      ':person_running_tone1:': { shortnames: [':runner_tone1:'] },
+      ':person_running_tone5:': { shortnames: [':runner_tone5:'] },
+      ':smile:': {}
+    },
+    altShortNames: {
+      ':+1:': ':thumbsup:',
+      ':+1_tone1:': ':thumbsup_tone1:',
+      ':runner:': ':person_running:',
+      ':runner_tone1:': ':person_running_tone1:'
+    }
+  };
+
+  // When default tone is active
+  adapters.applyEmojiSkinTone('default');
+  assert.equal(adapters.resolveEmojiShortcode('wave'), 'wave');
+  assert.equal(adapters.resolveEmojiShortcode('thumbsup'), 'thumbsup');
+  assert.equal(adapters.resolveEmojiShortcode('+1'), '+1');
+  assert.equal(adapters.resolveEmojiShortcode('runner'), 'runner');
+  assert.equal(adapters.resolveEmojiShortcode('smile'), 'smile');
+
+  // When tone1 is active
+  adapters.applyEmojiSkinTone('tone1');
+  assert.equal(adapters.resolveEmojiShortcode('wave'), 'wave_tone1');
+  assert.equal(adapters.resolveEmojiShortcode('thumbsup'), 'thumbsup_tone1');
+  // Aliases canonicalize to JoyPixels canonical keys with tone
+  assert.equal(adapters.resolveEmojiShortcode('+1'), 'thumbsup_tone1');
+  assert.equal(adapters.resolveEmojiShortcode('runner'), 'person_running_tone1');
+  // Emoji that does NOT support tone is untouched
+  assert.equal(adapters.resolveEmojiShortcode('smile'), 'smile');
+  // Emoji with existing tone modifier is untouched
+  assert.equal(adapters.resolveEmojiShortcode('wave_tone3'), 'wave_tone3');
+  assert.equal(adapters.resolveEmojiShortcode('+1_tone3'), '+1_tone3');
+  assert.equal(adapters.resolveEmojiShortcode('runner_tone2'), 'runner_tone2');
+
+  // When tone5 is active
+  adapters.applyEmojiSkinTone('tone5');
+  assert.equal(adapters.resolveEmojiShortcode('wave'), 'wave_tone5');
+  assert.equal(adapters.resolveEmojiShortcode('thumbsup'), 'thumbsup_tone5');
+  assert.equal(adapters.resolveEmojiShortcode('+1'), 'thumbsup_tone5');
+  assert.equal(adapters.resolveEmojiShortcode('runner'), 'person_running_tone5');
+  assert.equal(adapters.resolveEmojiShortcode('smile'), 'smile');
+});
+
+test('applyMermaid handles auto theme and look style', async () => {
+  const { context, adapters } = createAdapterContext();
+
+  context.document.documentElement.setAttribute('data-theme', 'dark');
+  await adapters.applyMermaid({ theme: 'auto', look: 'handDrawn' });
+
+  let config = adapters.getMermaidConfig();
+  assert.equal(config.theme, 'dark');
+  assert.equal(config.look, 'handDrawn');
+
+  context.document.documentElement.setAttribute('data-theme', 'light');
+  adapters.onAppThemeChange('light');
+  config = adapters.getMermaidConfig();
+  assert.equal(config.theme, 'default');
+  assert.equal(config.look, 'handDrawn');
+
+  // Explicit theme overrides auto
+  await adapters.applyMermaid({ theme: 'forest', look: 'classic' });
+  config = adapters.getMermaidConfig();
+  assert.equal(config.theme, 'forest');
+  assert.equal(config.look, 'classic');
+});
+
+test('applyHighlightTheme manages stylesheet, auto theme, and export styling', async () => {
+  const { context, adapters } = createAdapterContext();
+
+  context.document.documentElement.setAttribute('data-theme', 'light');
+  await adapters.applyHighlightTheme('monokai');
+  let link = context.document.getElementById('mdv-hljs-theme-link');
+  assert.ok(link);
+  assert.ok(link.href.includes('monokai.min.css'));
+  assert.equal(context.document.documentElement.getAttribute('data-custom-syntax-theme'), 'monokai');
+  assert.equal(context.document.documentElement.getAttribute('data-hljs-theme'), 'monokai');
+
+  // Test getHljsTheme and resolveHljsThemeName
+  assert.equal(adapters.resolveHljsThemeName('monokai'), 'monokai');
+  assert.equal(adapters.resolveHljsThemeName('auto', 'dark'), 'github-dark');
+  assert.equal(adapters.resolveHljsThemeName('auto', 'light'), 'github');
+  assert.equal(adapters.getHljsTheme(), 'monokai');
+
+  await adapters.applyHighlightTheme('auto');
+  link = context.document.getElementById('mdv-hljs-theme-link');
+  assert.ok(link.href.includes('github.min.css'));
+  assert.equal(adapters.getHljsTheme(), 'github');
+  assert.equal(adapters.getHljsTheme('dark'), 'github-dark');
+  assert.equal(context.document.documentElement.getAttribute('data-custom-syntax-theme'), null);
+  assert.equal(context.document.documentElement.getAttribute('data-hljs-theme'), 'auto');
+
+  // Verify mdv-hljs-style-helper content scopes preview and provides export rules
+  const helper = context.document.getElementById('mdv-hljs-style-helper');
+  assert.ok(helper);
+  assert.ok(helper.textContent.includes('.markdown-body:not(.pdf-export):not(.export-render-root)'));
+  assert.ok(helper.textContent.includes('.pdf-export.theme-dark'));
+  assert.ok(helper.textContent.includes('.pdf-export.theme-light'));
+  assert.ok(helper.textContent.includes('data-browser-print-export="dark"'));
+
+  context.document.documentElement.setAttribute('data-theme', 'dark');
+  adapters.onAppThemeChange('dark');
+  link = context.document.getElementById('mdv-hljs-theme-link');
+  assert.ok(link.href.includes('github-dark.min.css'));
+  assert.equal(adapters.getHljsTheme(), 'github-dark');
+  assert.equal(adapters.getHljsTheme('light'), 'github');
+});
+
+test('applyMathFont maintains MathJax 3 and 4 configuration compatibility without chtml.font string', async () => {
+  const { context, adapters } = createAdapterContext();
+
+  // Test when MathJax is not yet loaded
+  context.window.MathJax = {};
+  await adapters.applyMathFont('mathjax-stix2');
+
+  assert.equal(context.window.MathJax.config.output.font, 'mathjax-stix2');
+  assert.equal(typeof context.window.MathJax.config.chtml.font, 'undefined');
+
+  // Test when MathJax 3 is present (has version 3.x and no modular fonts)
+  context.window.MathJax = {
+    version: '3.2.2',
+    config: {
+      output: {},
+      chtml: {}
+    },
+    startup: {
+      getOutputJax: () => ({ name: 'chtml' })
+    }
+  };
+
+  await adapters.applyMathFont('mathjax-modern');
+  assert.equal(typeof context.window.MathJax.config.chtml.font, 'undefined');
+  assert.equal(context.window.MathJax.config.output.font, 'mathjax-modern');
+});
+
+test('applyMathFont initializes complete font resource configuration in MathJax 4', async () => {
+  const { context, adapters } = createAdapterContext();
+
+  const mockOutputJax = {
+    font: {
+      name: 'mathjax-modern',
+      options: {
+        fontURL: 'https://cdn.jsdelivr.net/npm/mathjax-modern-font/chtml/woff',
+        dynamicPrefix: '[mathjax-modern]/chtml/dynamic'
+      }
+    }
+  };
+
+  context.window.MathJax = {
+    version: '4.0.0-beta.7',
+    config: {},
+    loader: { paths: {} },
+    _: {
+      components: {
+        loader: {
+          CONFIG: {
+            paths: {
+              mathjax: 'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7'
+            }
+          }
+        }
+      },
+      output: {
+        fonts: {
+          'mathjax-stix2': {
+            chtml_ts: {
+              MathJaxStix2Font: class MockStixFont {}
+            }
+          }
+        }
+      }
+    },
+    startup: {
+      getOutputJax: () => mockOutputJax,
+      getDocument: () => ({})
+    }
+  };
+
+  await adapters.applyMathFont('mathjax-stix2');
+
+  const mj = context.window.MathJax;
+  assert.equal(mj.config.chtml.fontURL, 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font/chtml/woff');
+  assert.equal(mj.config.chtml.dynamicPrefix, '[mathjax-stix2]/chtml/dynamic');
+  assert.equal(mj.config.loader.paths['mathjax-stix2'], 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font');
+  assert.equal(mj.loader.paths['mathjax-stix2'], 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font');
+  assert.equal(mj._.components.loader.CONFIG.paths['mathjax-stix2'], 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font');
+  assert.equal(mockOutputJax.font.options.fontURL, 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font/chtml/woff');
+  assert.equal(mockOutputJax.font.options.dynamicPrefix, '[mathjax-stix2]/chtml/dynamic');
+});
+
+test('applyMathFont loads Modern font resources when resetting after alternate-font boot', async () => {
+  const { context, adapters } = createAdapterContext();
+
+  const mockOutputJax = {
+    font: {
+      name: 'mathjax-stix2',
+      options: {
+        fontURL: 'https://cdn.jsdelivr.net/npm/mathjax-stix2-font/chtml/woff',
+        dynamicPrefix: '[mathjax-stix2]/chtml/dynamic'
+      }
+    }
+  };
+
+  context.window.MathJax = {
+    version: '4.0.0-beta.7',
+    config: {},
+    loader: { paths: {} },
+    _: {
+      components: {
+        loader: {
+          CONFIG: {
+            paths: {
+              mathjax: 'https://cdn.jsdelivr.net/npm/mathjax@4.0.0-beta.7'
+            }
+          }
+        }
+      },
+      output: {
+        fonts: {
+          'mathjax-stix2': {
+            chtml_ts: {
+              MathJaxStix2Font: class MockStixFont {}
+            }
+          }
+        }
+      }
+    },
+    startup: {
+      getOutputJax: () => mockOutputJax,
+      getDocument: () => ({})
+    }
+  };
+
+  const origCreateElement = context.document.createElement.bind(context.document);
+  context.document.createElement = function(tag) {
+    const el = origCreateElement(tag);
+    if (tag.toLowerCase() === 'script') {
+      let _src = '';
+      Object.defineProperty(el, 'src', {
+        get() { return _src; },
+        set(val) {
+          _src = val;
+          if (val.includes('mathjax-modern-font')) {
+            context.window.MathJax._.output.fonts['mathjax-modern'] = {
+              chtml_ts: {
+                MathJaxModernFont: class MockModernFont {}
+              }
+            };
+          }
+          setTimeout(() => { if (this.onload) this.onload(); }, 1);
+        }
+      });
+    }
+    return el;
+  };
+
+  await adapters.applyMathFont('mathjax-modern');
+
+  const mj = context.window.MathJax;
+  assert.equal(mj.config.output.font, 'mathjax-modern');
+  assert.equal(mj.config.chtml.fontURL, 'https://cdn.jsdelivr.net/npm/mathjax-modern-font/chtml/woff');
+  assert.equal(mj.config.chtml.dynamicPrefix, '[mathjax-modern]/chtml/dynamic');
+  assert.equal(mj.config.loader.paths['mathjax-modern'], 'https://cdn.jsdelivr.net/npm/mathjax-modern-font');
+  assert.equal(mockOutputJax.font.options.fontURL, 'https://cdn.jsdelivr.net/npm/mathjax-modern-font/chtml/woff');
+  assert.equal(mockOutputJax.font.options.dynamicPrefix, '[mathjax-modern]/chtml/dynamic');
+  assert.ok(mj._.output.fonts['mathjax-modern']);
+});
+
diff --git a/tests/unit/visual-settings-store.test.mjs b/tests/unit/visual-settings-store.test.mjs
new file mode 100644
index 00000000..23e0fc53
--- /dev/null
+++ b/tests/unit/visual-settings-store.test.mjs
@@ -0,0 +1,112 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+// Mock localStorage and window in Node environment
+const mockStorage = new Map();
+global.localStorage = {
+  getItem: (key) => (mockStorage.has(key) ? mockStorage.get(key) : null),
+  setItem: (key, val) => mockStorage.set(key, String(val)),
+  removeItem: (key) => mockStorage.delete(key),
+  clear: () => mockStorage.clear()
+};
+global.window = {};
+
+// Import store
+const { createRequire } = await import('node:module');
+const require = createRequire(import.meta.url);
+const store = require('../../visual-settings-store.js');
+
+test('visualSettingsStore - returns initial defaults', () => {
+  mockStorage.clear();
+  store.resetSettings();
+  const settings = store.getSettings();
+  assert.equal(settings.hljsTheme, 'auto');
+  assert.equal(settings.mathFont, 'mathjax-modern');
+  assert.equal(settings.mermaidTheme, 'auto');
+  assert.equal(settings.mermaidLook, 'classic');
+  assert.equal(settings.emojiSkinTone, 'default');
+});
+
+test('visualSettingsStore - updateSetting updates value, persists, and notifies listeners', () => {
+  mockStorage.clear();
+  store.resetSettings();
+
+  let changeNotified = false;
+  let notifiedKey = null;
+  let notifiedVal = null;
+  let notifiedSettings = null;
+
+  const unsubscribe = store.onSettingsChange((settings, key, val) => {
+    changeNotified = true;
+    notifiedKey = key;
+    notifiedVal = val;
+    notifiedSettings = settings;
+  });
+
+  const success = store.updateSetting('hljsTheme', 'monokai');
+  assert.equal(success, true);
+  assert.equal(store.getSettings().hljsTheme, 'monokai');
+  assert.equal(changeNotified, true);
+  assert.equal(notifiedKey, 'hljsTheme');
+  assert.equal(notifiedVal, 'monokai');
+  assert.equal(notifiedSettings.hljsTheme, 'monokai');
+
+  // Verify saved in localStorage
+  const saved = JSON.parse(mockStorage.get(store.STORAGE_KEY));
+  assert.equal(saved.hljsTheme, 'monokai');
+
+  unsubscribe();
+});
+
+test('visualSettingsStore - rejects invalid values or unknown keys', () => {
+  store.resetSettings();
+
+  // Invalid value
+  const badValSuccess = store.updateSetting('mermaidTheme', 'nonexistent-theme');
+  assert.equal(badValSuccess, false);
+  assert.equal(store.getSettings().mermaidTheme, 'auto');
+
+  // Unknown key
+  const badKeySuccess = store.updateSetting('unknownKey', 'something');
+  assert.equal(badKeySuccess, false);
+});
+
+test('visualSettingsStore - resetSettings restores defaults and saves', () => {
+  store.updateSetting('mermaidLook', 'handDrawn');
+  store.updateSetting('emojiSkinTone', 'tone3');
+  assert.equal(store.getSettings().mermaidLook, 'handDrawn');
+  assert.equal(store.getSettings().emojiSkinTone, 'tone3');
+
+  let resetNotified = false;
+  const unsub = store.onSettingsChange((settings, key) => {
+    if (key === '*') resetNotified = true;
+  });
+
+  const resetResult = store.resetSettings();
+  assert.equal(resetResult.mermaidLook, 'classic');
+  assert.equal(resetResult.emojiSkinTone, 'default');
+  assert.equal(resetNotified, true);
+
+  const saved = JSON.parse(mockStorage.get(store.STORAGE_KEY));
+  assert.equal(saved.mermaidLook, 'classic');
+  assert.equal(saved.emojiSkinTone, 'default');
+
+  unsub();
+});
+
+test('visualSettingsStore - validation recovers from corrupted or invalid localStorage data', () => {
+  mockStorage.set(store.STORAGE_KEY, JSON.stringify({
+    hljsTheme: 'invalid-hljs',
+    mathFont: 'mathjax-stix2',
+    mermaidTheme: 'invalid-mermaid',
+    corruptKey: 123
+  }));
+
+  // Re-run validation
+  const validated = store.validateSettings(JSON.parse(mockStorage.get(store.STORAGE_KEY)));
+  assert.equal(validated.hljsTheme, 'auto'); // Fell back to default
+  assert.equal(validated.mathFont, 'mathjax-stix2'); // Kept valid value
+  assert.equal(validated.mermaidTheme, 'auto'); // Fell back to default
+  assert.equal(validated.mermaidLook, 'classic'); // Restored missing key
+  assert.equal(validated.emojiSkinTone, 'default'); // Restored missing key
+});
diff --git a/visual-settings-adapters.js b/visual-settings-adapters.js
new file mode 100644
index 00000000..bf05604f
--- /dev/null
+++ b/visual-settings-adapters.js
@@ -0,0 +1,851 @@
+/**
+ * visual-settings-adapters.js
+ * Library adapters for Markdown Viewer visual settings (Issue #129).
+ * Manages runtime theming and visual configurations for:
+ * - highlight.js: theme stylesheets (auto, github, monokai, dracula, etc.)
+ * - MathJax: mathematical typesetting fonts (Modern, STIX2, Asana, Bonum, etc.)
+ * - Mermaid: diagram themes (auto, default, dark, forest, neutral, base) & looks (classic, handDrawn)
+ * - JoyPixels: emoji Fitzpatrick skin tones (default, tone1 - tone5)
+ */
+
+(function () {
+  'use strict';
+
+  // Allowed options metadata
+  const OPTIONS = Object.freeze({
+    hljsTheme: [
+      { value: 'auto', label: 'Auto (Follow App)' },
+      { value: 'github', label: 'GitHub' },
+      { value: 'github-dark', label: 'GitHub Dark' },
+      { value: 'monokai', label: 'Monokai' },
+      { value: 'dracula', label: 'Dracula' },
+      { value: 'atom-one-dark', label: 'Atom One Dark' },
+      { value: 'atom-one-light', label: 'Atom One Light' },
+      { value: 'solarized-dark', label: 'Solarized Dark' },
+      { value: 'solarized-light', label: 'Solarized Light' },
+      { value: 'vs', label: 'Visual Studio' },
+      { value: 'vs2015', label: 'Visual Studio 2015' },
+      { value: 'nord', label: 'Nord' },
+      { value: 'tokyo-night-dark', label: 'Tokyo Night Dark' },
+      { value: 'tokyo-night-light', label: 'Tokyo Night Light' },
+      { value: 'default', label: 'Default' }
+    ],
+    mathFont: [
+      { value: 'mathjax-modern', label: 'Computer Modern (TeX)' },
+      { value: 'mathjax-tex', label: 'TeX (Classic)' },
+      { value: 'mathjax-stix2', label: 'STIX Two' },
+      { value: 'mathjax-asana', label: 'Asana Math' },
+      { value: 'mathjax-bonum', label: 'Gyre Bonum' },
+      { value: 'mathjax-dejavu', label: 'DejaVu Math' },
+      { value: 'mathjax-fira', label: 'Fira Math' },
+      { value: 'mathjax-pagella', label: 'Gyre Pagella' },
+      { value: 'mathjax-schola', label: 'Gyre Schola' },
+      { value: 'mathjax-termes', label: 'Gyre Termes' }
+    ],
+    mermaidTheme: [
+      { value: 'auto', label: 'Auto (Follow App)' },
+      { value: 'default', label: 'Default' },
+      { value: 'neutral', label: 'Neutral' },
+      { value: 'dark', label: 'Dark' },
+      { value: 'forest', label: 'Forest' },
+      { value: 'base', label: 'Base' }
+    ],
+    mermaidLook: [
+      { value: 'classic', label: 'Classic' },
+      { value: 'handDrawn', label: 'Hand Drawn' }
+    ],
+    emojiSkinTone: [
+      { value: 'default', label: 'Default (Yellow)' },
+      { value: 'tone1', label: 'Light (Tone 1)' },
+      { value: 'tone2', label: 'Medium-Light (Tone 2)' },
+      { value: 'tone3', label: 'Medium (Tone 3)' },
+      { value: 'tone4', label: 'Medium-Dark (Tone 4)' },
+      { value: 'tone5', label: 'Dark (Tone 5)' }
+    ]
+  });
+
+  const DEFAULTS = Object.freeze({
+    hljsTheme: 'auto',
+    mathFont: 'mathjax-modern',
+    mermaidTheme: 'auto',
+    mermaidLook: 'classic',
+    emojiSkinTone: 'default'
+  });
+
+  // Active state
+  let _activeHljsTheme = DEFAULTS.hljsTheme;
+  let _activeMathFont = DEFAULTS.mathFont;
+  let _activeMermaidTheme = DEFAULTS.mermaidTheme;
+  let _activeMermaidLook = DEFAULTS.mermaidLook;
+  let _activeEmojiSkinTone = DEFAULTS.emojiSkinTone;
+
+  const HLJS_CDN_BASE = 'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/styles/';
+  const MATHJAX_FONT_CDN_BASE = 'https://cdn.jsdelivr.net/npm/';
+
+  // Mapping for highlight.js themes on cdnjs 11.9.0 (dracula & solarized live under base16/)
+  const HLJS_THEME_PATHS = Object.freeze({
+    'github': 'github.min.css',
+    'github-dark': 'github-dark.min.css',
+    'monokai': 'monokai.min.css',
+    'dracula': 'base16/dracula.min.css',
+    'atom-one-dark': 'atom-one-dark.min.css',
+    'atom-one-light': 'atom-one-light.min.css',
+    'solarized-dark': 'base16/solarized-dark.min.css',
+    'solarized-light': 'base16/solarized-light.min.css',
+    'vs': 'vs.min.css',
+    'vs2015': 'vs2015.min.css',
+    'nord': 'nord.min.css',
+    'tokyo-night-dark': 'tokyo-night-dark.min.css',
+    'tokyo-night-light': 'tokyo-night-light.min.css',
+    'default': 'default.min.css'
+  });
+
+  // Fallback background and foreground colors for syntax themes
+  const HLJS_THEME_COLORS = Object.freeze({
+    'github': { bg: '#ffffff', color: '#24292e' },
+    'github-dark': { bg: '#0d1117', color: '#c9d1d9' },
+    'monokai': { bg: '#272822', color: '#dddddd' },
+    'dracula': { bg: '#282a36', color: '#f8f8f2' },
+    'atom-one-dark': { bg: '#282c34', color: '#abb2bf' },
+    'atom-one-light': { bg: '#fafafa', color: '#383a42' },
+    'solarized-dark': { bg: '#002b36', color: '#839496' },
+    'solarized-light': { bg: '#fdf6e3', color: '#657b83' },
+    'vs': { bg: '#ffffff', color: '#000000' },
+    'vs2015': { bg: '#1e1e1e', color: '#dcdcdc' },
+    'nord': { bg: '#2e3440', color: '#d8dee9' },
+    'tokyo-night-dark': { bg: '#1a1b26', color: '#a9b1d6' },
+    'tokyo-night-light': { bg: '#d5d6db', color: '#343b58' },
+    'default': { bg: '#f3f3f3', color: '#444444' }
+  });
+
+  function getEffectiveAppTheme() {
+    if (typeof document === 'undefined' || !document.documentElement) return 'light';
+    return document.documentElement.getAttribute('data-theme') === 'dark' ? 'dark' : 'light';
+  }
+
+  function resolveHljsThemeName(theme, themeOverride) {
+    if (theme === 'auto') {
+      const effective = themeOverride || getEffectiveAppTheme();
+      return effective === 'dark' ? 'github-dark' : 'github';
+    }
+    return theme || 'github';
+  }
+
+  function resolveMermaidThemeName(theme, themeOverride) {
+    if (theme === 'auto') {
+      const effective = themeOverride || getEffectiveAppTheme();
+      return effective === 'dark' ? 'dark' : 'default';
+    }
+    return theme || 'default';
+  }
+
+  // Update dynamic helper stylesheet so code blocks adopt the theme's background and contrast
+  function updateHljsHelperStyle(effectiveName) {
+    if (typeof document === 'undefined') return;
+    let style = document.getElementById('mdv-hljs-style-helper');
+    if (!style) {
+      style = document.createElement('style');
+      style.id = 'mdv-hljs-style-helper';
+      if (document.head) document.head.appendChild(style);
+    }
+
+    const fallback = HLJS_THEME_COLORS[effectiveName] || HLJS_THEME_COLORS['github'];
+    let bg = fallback.bg;
+    let fg = fallback.color;
+
+    if (document.body && typeof window !== 'undefined' && typeof window.getComputedStyle === 'function') {
+      try {
+        const probe = document.createElement('div');
+        probe.className = 'hljs';
+        probe.style.position = 'fixed';
+        probe.style.top = '-9999px';
+        probe.style.left = '-9999px';
+        probe.style.visibility = 'hidden';
+        document.body.appendChild(probe);
+        const cs = window.getComputedStyle(probe);
+        if (cs && cs.backgroundColor && cs.backgroundColor !== 'rgba(0, 0, 0, 0)' && cs.backgroundColor !== 'transparent') {
+          bg = cs.backgroundColor;
+        }
+        if (cs && cs.color && cs.color !== 'rgba(0, 0, 0, 0)' && cs.color !== 'transparent') {
+          fg = cs.color;
+        }
+        document.body.removeChild(probe);
+      } catch (_) {}
+    }
+
+    if (_activeHljsTheme === 'auto') {
+      style.textContent = `
+        /* Preview code blocks in Auto mode adopt application theme */
+        .markdown-body:not(.pdf-export):not(.export-render-root) pre:has(> code.hljs) {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .markdown-body:not(.pdf-export):not(.export-render-root) pre > code.hljs,
+        .markdown-body:not(.pdf-export):not(.export-render-root) code.hljs {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        /* Export documents in Auto mode resolve against the selected export theme */
+        .pdf-export.theme-dark pre:has(> code.hljs),
+        .pdf-export[data-theme="dark"] pre:has(> code.hljs),
+        .export-render-root[data-theme="dark"] pre:has(> code.hljs),
+        .export-render-root.theme-dark pre:has(> code.hljs) {
+          background-color: #161b22 !important;
+          color: #c9d1d9 !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .pdf-export.theme-dark pre > code.hljs,
+        .pdf-export[data-theme="dark"] pre > code.hljs,
+        .export-render-root[data-theme="dark"] pre > code.hljs,
+        .export-render-root.theme-dark pre > code.hljs,
+        .pdf-export.theme-dark code.hljs,
+        .pdf-export[data-theme="dark"] code.hljs,
+        .export-render-root[data-theme="dark"] code.hljs,
+        .export-render-root.theme-dark code.hljs {
+          background-color: #161b22 !important;
+          color: #c9d1d9 !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        .pdf-export.theme-light pre:has(> code.hljs),
+        .pdf-export[data-theme="light"] pre:has(> code.hljs),
+        .export-render-root[data-theme="light"] pre:has(> code.hljs),
+        .export-render-root.theme-light pre:has(> code.hljs) {
+          background-color: #f6f8fa !important;
+          color: #24292e !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .pdf-export.theme-light pre > code.hljs,
+        .pdf-export[data-theme="light"] pre > code.hljs,
+        .export-render-root[data-theme="light"] pre > code.hljs,
+        .export-render-root.theme-light pre > code.hljs,
+        .pdf-export.theme-light code.hljs,
+        .pdf-export[data-theme="light"] code.hljs,
+        .export-render-root[data-theme="light"] code.hljs,
+        .export-render-root.theme-light code.hljs {
+          background-color: #f6f8fa !important;
+          color: #24292e !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+
+        @media print {
+          .markdown-body:not(.pdf-export):not(.export-render-root) pre:has(> code.hljs),
+          .markdown-body:not(.pdf-export):not(.export-render-root) pre > code.hljs,
+          .markdown-body:not(.pdf-export):not(.export-render-root) code.hljs {
+            background-color: ${bg} !important;
+            color: ${fg} !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+
+          html[data-browser-print-export="dark"] .markdown-body pre:has(> code.hljs),
+          html[data-browser-print-export="dark"] .markdown-body pre > code.hljs,
+          html[data-browser-print-export="dark"] .markdown-body code.hljs,
+          .pdf-export.theme-dark pre:has(> code.hljs),
+          .pdf-export.theme-dark pre > code.hljs,
+          .pdf-export.theme-dark code.hljs {
+            background-color: #161b22 !important;
+            color: #c9d1d9 !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light pre:has(> code.hljs),
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light pre > code.hljs,
+          html:not([data-browser-print-export="dark"]) .markdown-body.pdf-export.theme-light code.hljs,
+          html[data-browser-print-export="light"] .markdown-body pre:has(> code.hljs),
+          html[data-browser-print-export="light"] .markdown-body pre > code.hljs,
+          html[data-browser-print-export="light"] .markdown-body code.hljs,
+          .pdf-export.theme-light pre:has(> code.hljs),
+          .pdf-export.theme-light pre > code.hljs,
+          .pdf-export.theme-light code.hljs {
+            background-color: #f6f8fa !important;
+            color: #24292e !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+        }
+      `;
+    } else {
+      style.textContent = `
+        .markdown-body pre:has(> code.hljs) {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          border-radius: 6px;
+          padding: 16px !important;
+          overflow: auto;
+        }
+        .markdown-body pre > code.hljs,
+        .markdown-body code.hljs {
+          background-color: ${bg} !important;
+          color: ${fg} !important;
+          padding: 0 !important;
+          border-radius: 0 !important;
+        }
+        @media print {
+          .markdown-body pre:has(> code.hljs),
+          .markdown-body pre > code.hljs,
+          .markdown-body code.hljs {
+            background-color: ${bg} !important;
+            color: ${fg} !important;
+            -webkit-print-color-adjust: exact !important;
+            print-color-adjust: exact !important;
+          }
+        }
+      `;
+    }
+  }
+
+  /**
+   * Apply highlight.js theme stylesheet.
+   * Dynamically manages the <link id="mdv-hljs-theme-link"> in <head>.
+   */
+  function applyHighlightTheme(theme) {
+    _activeHljsTheme = theme || DEFAULTS.hljsTheme;
+    const effectiveName = resolveHljsThemeName(_activeHljsTheme);
+    updateHljsHelperStyle(effectiveName);
+
+    if (typeof document === 'undefined') return Promise.resolve();
+
+    if (document.documentElement) {
+      if (_activeHljsTheme !== 'auto') {
+        document.documentElement.setAttribute('data-custom-syntax-theme', _activeHljsTheme);
+        document.documentElement.setAttribute('data-hljs-theme', _activeHljsTheme);
+      } else {
+        if (typeof document.documentElement.removeAttribute === 'function') {
+          document.documentElement.removeAttribute('data-custom-syntax-theme');
+        }
+        document.documentElement.setAttribute('data-hljs-theme', 'auto');
+      }
+    }
+
+    const subpath = HLJS_THEME_PATHS[effectiveName] || `${effectiveName}.min.css`;
+    const href = `${HLJS_CDN_BASE}${subpath}`;
+
+    let link = document.getElementById('mdv-hljs-theme-link');
+    if (!link) {
+      link = document.createElement('link');
+      link.id = 'mdv-hljs-theme-link';
+      link.rel = 'stylesheet';
+      link.crossOrigin = 'anonymous';
+      if (document.head) document.head.appendChild(link);
+    }
+
+    if (link.getAttribute('href') === href) {
+      updateHljsHelperStyle(effectiveName);
+      return Promise.resolve();
+    }
+
+    return new Promise((resolve) => {
+      link.onload = () => {
+        updateHljsHelperStyle(effectiveName);
+        resolve();
+      };
+      link.onerror = () => {
+        console.warn(`[VisualAdapters] Failed to load highlight.js theme: ${href}`);
+        updateHljsHelperStyle(effectiveName);
+        resolve(); // resolve gracefully so UI does not stall
+      };
+      link.href = href;
+    });
+  }
+
+  /**
+   * Dynamically loads a script tag if not already loaded.
+   */
+  const _loadedScripts = new Set();
+  function loadScriptOnce(src) {
+    if (_loadedScripts.has(src)) return Promise.resolve();
+    if (typeof document === 'undefined') return Promise.resolve();
+    return new Promise((resolve, reject) => {
+      const existing = document.querySelector(`script[src="${src}"]`);
+      if (existing) {
+        _loadedScripts.add(src);
+        return resolve();
+      }
+      const script = document.createElement('script');
+      script.src = src;
+      script.charset = 'UTF-8';
+      script.crossOrigin = 'anonymous';
+      script.onload = () => {
+        _loadedScripts.add(src);
+        resolve();
+      };
+      script.onerror = (err) => {
+        console.warn(`[VisualAdapters] Script load failed: ${src}`, err);
+        reject(err);
+      };
+      if (document.head) document.head.appendChild(script);
+      else if (document.body) document.body.appendChild(script);
+    });
+  }
+
+  /**
+   * Apply MathJax mathematical font.
+   */
+  async function applyMathFont(font) {
+    _activeMathFont = font || DEFAULTS.mathFont;
+
+    if (typeof window === 'undefined' || !window.MathJax) {
+      return;
+    }
+
+    const isMathJax3 = (window.MathJax.version && String(window.MathJax.version).startsWith('3.')) ||
+                       (window.MathJax._ && !window.MathJax._.output?.fonts);
+
+    // Configure font and complete resource configuration in MathJax
+    const fontPkgUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font`;
+    const fontWoffUrl = `${fontPkgUrl}/chtml/woff`;
+    const fontDynamicPrefix = `[${_activeMathFont}]/chtml/dynamic`;
+
+    window.MathJax.output = window.MathJax.output || {};
+    window.MathJax.output.font = _activeMathFont;
+
+    window.MathJax.config = window.MathJax.config || {};
+    window.MathJax.config.output = window.MathJax.config.output || {};
+    window.MathJax.config.output.font = _activeMathFont;
+
+    window.MathJax.chtml = window.MathJax.chtml || {};
+    window.MathJax.config.chtml = window.MathJax.config.chtml || {};
+    if (typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
+    if (typeof window.MathJax.config.chtml.font === 'string') {
+      delete window.MathJax.config.chtml.font;
+    }
+
+    if (!isMathJax3) {
+      window.MathJax.chtml.fontURL = fontWoffUrl;
+      window.MathJax.chtml.dynamicPrefix = fontDynamicPrefix;
+      window.MathJax.config.chtml.fontURL = fontWoffUrl;
+      window.MathJax.config.chtml.dynamicPrefix = fontDynamicPrefix;
+
+      window.MathJax.loader = window.MathJax.loader || {};
+      window.MathJax.loader.paths = window.MathJax.loader.paths || {};
+      window.MathJax.loader.paths[_activeMathFont] = fontPkgUrl;
+
+      window.MathJax.config.loader = window.MathJax.config.loader || {};
+      window.MathJax.config.loader.paths = window.MathJax.config.loader.paths || {};
+      window.MathJax.config.loader.paths[_activeMathFont] = fontPkgUrl;
+
+      if (window.MathJax._ && window.MathJax._.components && window.MathJax._.components.loader && window.MathJax._.components.loader.CONFIG && window.MathJax._.components.loader.CONFIG.paths) {
+        window.MathJax._.components.loader.CONFIG.paths[_activeMathFont] = fontPkgUrl;
+      }
+    }
+
+    // Load external font script if needed and MathJax core is already loaded.
+    // If MathJax has not loaded yet, configuring output.font and resource URLs
+    // is sufficient because MathJax's startup loader will load the font component itself.
+    const fontAlreadyLoaded = !!(
+      window.MathJax._ &&
+      window.MathJax._.output &&
+      window.MathJax._.output.fonts &&
+      window.MathJax._.output.fonts[_activeMathFont]
+    );
+
+    if (!fontAlreadyLoaded && window.MathJax._ && !isMathJax3) {
+      const fontUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font/chtml.js`;
+      try {
+        await loadScriptOnce(fontUrl);
+      } catch (err) {
+        console.warn(`[VisualAdapters] MathJax font load failed (${_activeMathFont}), falling back to bundled Modern:`, err);
+        _activeMathFont = 'mathjax-modern';
+        if (
+          window.MathJax._ &&
+          (!window.MathJax._.output || !window.MathJax._.output.fonts || !window.MathJax._.output.fonts['mathjax-modern'])
+        ) {
+          try {
+            await loadScriptOnce(`${MATHJAX_FONT_CDN_BASE}mathjax-modern-font/chtml.js`);
+          } catch (modernErr) {
+            console.warn('[VisualAdapters] MathJax modern fallback load failed:', modernErr);
+          }
+        }
+      }
+    }
+
+    // Extract registered font class and activate its fontData
+    let fontClass = null;
+    if (window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts) {
+      const fontPkg = window.MathJax._.output.fonts[_activeMathFont];
+      if (fontPkg && fontPkg.chtml_ts) {
+        fontClass = Object.values(fontPkg.chtml_ts)[0] || null;
+      }
+    }
+
+    // If font class was not found (e.g. offline fallback), fall back to bundled modern font
+    if (!fontClass && _activeMathFont !== 'mathjax-modern' && window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts && window.MathJax._.output.fonts['mathjax-modern']) {
+      _activeMathFont = 'mathjax-modern';
+      const modernPkg = window.MathJax._.output.fonts['mathjax-modern'];
+      if (modernPkg.chtml_ts) {
+        fontClass = Object.values(modernPkg.chtml_ts)[0] || null;
+      }
+    }
+
+    // Update MathJax configuration with effective font and complete resource configuration
+    const effectivePkgUrl = `${MATHJAX_FONT_CDN_BASE}${_activeMathFont}-font`;
+    const effectiveWoffUrl = `${effectivePkgUrl}/chtml/woff`;
+    const effectiveDynamicPrefix = `[${_activeMathFont}]/chtml/dynamic`;
+
+    window.MathJax.config.output.font = _activeMathFont;
+    if (typeof window.MathJax.config.chtml.font === 'string') {
+      delete window.MathJax.config.chtml.font;
+    }
+    if (typeof window.MathJax.chtml.font === 'string') {
+      delete window.MathJax.chtml.font;
+    }
+    window.MathJax.config.chtml.fontURL = effectiveWoffUrl;
+    window.MathJax.config.chtml.dynamicPrefix = effectiveDynamicPrefix;
+    window.MathJax.chtml.fontURL = effectiveWoffUrl;
+    window.MathJax.chtml.dynamicPrefix = effectiveDynamicPrefix;
+
+    window.MathJax.config.loader = window.MathJax.config.loader || {};
+    window.MathJax.config.loader.paths = window.MathJax.config.loader.paths || {};
+    window.MathJax.config.loader.paths[_activeMathFont] = effectivePkgUrl;
+    window.MathJax.loader = window.MathJax.loader || {};
+    window.MathJax.loader.paths = window.MathJax.loader.paths || {};
+    window.MathJax.loader.paths[_activeMathFont] = effectivePkgUrl;
+    if (window.MathJax._ && window.MathJax._.components && window.MathJax._.components.loader && window.MathJax._.components.loader.CONFIG && window.MathJax._.components.loader.CONFIG.paths) {
+      window.MathJax._.components.loader.CONFIG.paths[_activeMathFont] = effectivePkgUrl;
+    }
+
+    if (fontClass) {
+      window.MathJax.config.chtml.fontData = fontClass;
+      window.MathJax.chtml.fontData = fontClass;
+    } else if (_activeMathFont === 'mathjax-modern' && window.MathJax._ && window.MathJax._.output && window.MathJax._.output.fonts && window.MathJax._.output.fonts['mathjax-modern']) {
+      const modernPkg = window.MathJax._.output.fonts['mathjax-modern'];
+      if (modernPkg.chtml_ts) {
+        window.MathJax.config.chtml.fontData = Object.values(modernPkg.chtml_ts)[0] || null;
+        window.MathJax.chtml.fontData = window.MathJax.config.chtml.fontData;
+      }
+    }
+
+    // If MathJax startup has completed, switch output jax, document and re-typeset
+    if (!isMathJax3 && window.MathJax.startup && typeof window.MathJax.startup.getOutputJax === 'function') {
+      try {
+        const newOutputJax = window.MathJax.startup.getOutputJax();
+        if (newOutputJax) {
+          if (newOutputJax.font && newOutputJax.font.options) {
+            newOutputJax.font.options.fontURL = effectiveWoffUrl;
+            newOutputJax.font.options.dynamicPrefix = effectiveDynamicPrefix;
+          }
+          window.MathJax.startup.output = newOutputJax;
+          if (typeof window.MathJax.startup.getDocument === 'function') {
+            window.MathJax.startup.document = window.MathJax.startup.getDocument();
+          }
+        }
+
+        // Re-render math in preview if present
+        if (typeof document !== 'undefined') {
+          const preview = document.getElementById('markdown-preview');
+          const splitPreview = document.getElementById('document-split-preview');
+          const roots = [];
+          if (preview && preview.querySelector('mjx-container, .math-block, .math-inline')) roots.push(preview);
+          if (splitPreview && splitPreview.querySelector('mjx-container, .math-block, .math-inline')) roots.push(splitPreview);
+
+          if (roots.length > 0) {
+            if (typeof window.renderMarkdown === 'function') {
+              window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+            } else if (typeof window.MathJax.typesetPromise === 'function') {
+              await window.MathJax.typesetPromise(roots);
+            }
+          }
+        }
+      } catch (err) {
+        console.warn('[VisualAdapters] MathJax font switch error:', err);
+      }
+    }
+  }
+
+  /**
+   * Apply Mermaid theme and look drawing style.
+   */
+  async function applyMermaid(options) {
+    if (options) {
+      if (typeof options.theme !== 'undefined') _activeMermaidTheme = options.theme;
+      if (typeof options.look !== 'undefined') _activeMermaidLook = options.look;
+    }
+
+    if (typeof mermaid === 'undefined') {
+      return;
+    }
+
+    const effectiveTheme = resolveMermaidThemeName(_activeMermaidTheme);
+    const effectiveLook = _activeMermaidLook === 'handDrawn' ? 'handDrawn' : 'classic';
+
+    try {
+      mermaid.initialize({
+        startOnLoad: false,
+        theme: effectiveTheme,
+        look: effectiveLook,
+        securityLevel: 'strict',
+        flowchart: { useMaxWidth: true, htmlLabels: true },
+        fontSize: 16,
+        gantt: { useWidth: 1200 }
+      });
+    } catch (err) {
+      console.warn('[VisualAdapters] Mermaid initialize error:', err);
+    }
+
+    // Re-render existing Mermaid nodes in the preview
+    if (typeof document !== 'undefined') {
+      const preview = document.getElementById('markdown-preview');
+      if (preview) {
+        const mermaidNodes = preview.querySelectorAll('.mermaid');
+        if (mermaidNodes.length > 0) {
+          mermaidNodes.forEach((node) => {
+            const originalCode = node.getAttribute('data-original-code');
+            if (originalCode) {
+              node.innerHTML = decodeURIComponent(originalCode)
+                .replace(/&/g, '&amp;')
+                .replace(/</g, '&lt;')
+                .replace(/>/g, '&gt;');
+            }
+            node.removeAttribute('data-processed');
+
+            const container = node.closest('.mermaid-container');
+            if (container) {
+              const oldToolbar = container.querySelector('.mermaid-toolbar');
+              if (oldToolbar) oldToolbar.remove();
+            }
+          });
+
+          if (typeof window.renderMarkdown === 'function') {
+            window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+          }
+        }
+      }
+    }
+  }
+
+  /**
+   * Resolves a shortcode to JoyPixels' canonical shortcode name (e.g. '+1' -> 'thumbsup', 'runner' -> 'person_running').
+   */
+  function getCanonicalEmojiShortcode(code, joy) {
+    if (!code) return code;
+    const clean = code.replace(/^:|:$/g, '');
+    if (!joy) return clean;
+    const colonCode = `:${clean}:`;
+
+    // 1. If it's already a canonical key in joy.emojiList
+    if (joy.emojiList && joy.emojiList[colonCode]) {
+      return clean;
+    }
+
+    // 2. Check joy.altShortNames (native JoyPixels alias map)
+    if (joy.altShortNames) {
+      const alt = joy.altShortNames[colonCode] || joy.altShortNames[clean];
+      if (alt) {
+        return alt.replace(/^:|:$/g, '');
+      }
+    }
+
+    // 3. Fallback: scan emojiList if altShortNames is not populated
+    if (joy.emojiList) {
+      for (const [key, val] of Object.entries(joy.emojiList)) {
+        if (val && Array.isArray(val.shortnames)) {
+          if (val.shortnames.includes(colonCode) || val.shortnames.includes(clean)) {
+            return key.replace(/^:|:$/g, '');
+          }
+        }
+      }
+    }
+
+    return clean;
+  }
+
+  /**
+   * Resolves an emoji shortcode to include the active Fitzpatrick skin tone modifier
+   * if the emoji supports skin tone modifiers, otherwise returns the original shortcode.
+   * Canonicalizes JoyPixels aliases prior to checking tone support.
+   * Shortcodes with explicit tone modifiers are left untouched.
+   */
+  function resolveEmojiShortcode(shortcode) {
+    if (!shortcode) return shortcode;
+    const clean = shortcode.replace(/^:|:$/g, '');
+
+    if (!_activeEmojiSkinTone || _activeEmojiSkinTone === 'default') {
+      return shortcode;
+    }
+
+    // If already has an explicit skin tone suffix, preserve it
+    if (/_tone[1-5]$/.test(clean)) {
+      return shortcode;
+    }
+
+    const joy = typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined);
+    const canonical = getCanonicalEmojiShortcode(clean, joy);
+    const toneCandidate = `${canonical}_${_activeEmojiSkinTone}`;
+
+    // Check JoyPixels library list if available
+    if (joy && joy.emojiList) {
+      if (joy.emojiList[`:${toneCandidate}:`]) {
+        return toneCandidate;
+      }
+      const directCandidate = `${clean}_${_activeEmojiSkinTone}`;
+      if (joy.emojiList[`:${directCandidate}:`]) {
+        return directCandidate;
+      }
+      return shortcode;
+    }
+
+    return toneCandidate;
+  }
+
+  /**
+   * Apply JoyPixels emoji skin tone.
+   */
+  function applyEmojiSkinTone(tone) {
+    _activeEmojiSkinTone = tone || DEFAULTS.emojiSkinTone;
+
+    const joy = typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined);
+    if (joy && typeof joy.shortnameToUnicode === 'function' && !joy._visualAdapterWrapped) {
+      const origShortnameToUnicode = joy.shortnameToUnicode.bind(joy);
+      joy.shortnameToUnicode = function (str) {
+        if (_activeEmojiSkinTone && _activeEmojiSkinTone !== 'default') {
+          str = str.replace(/:([\w+-]+):/g, function (match, code) {
+            const resolved = resolveEmojiShortcode(code);
+            return `:${resolved}:`;
+          });
+        }
+        return origShortnameToUnicode(str);
+      };
+      joy._visualAdapterWrapped = true;
+    }
+
+    if (typeof document !== 'undefined') {
+      const preview = document.getElementById('markdown-preview');
+      if (preview && typeof window.renderMarkdown === 'function') {
+        window.renderMarkdown({ force: true, forceAdvancedPostProcess: true });
+      }
+    }
+  }
+
+  /**
+   * React to application light/dark theme toggle.
+   * Idempotently re-applies any setting configured as 'auto'.
+   */
+  function onAppThemeChange(newAppTheme) {
+    if (_activeHljsTheme === 'auto') {
+      applyHighlightTheme('auto');
+    }
+    if (_activeMermaidTheme === 'auto') {
+      applyMermaid({ theme: 'auto', look: _activeMermaidLook });
+    }
+  }
+
+  // Observe theme changes on documentElement
+  if (typeof MutationObserver !== 'undefined' && typeof document !== 'undefined' && document.documentElement) {
+    const themeObserver = new MutationObserver((mutations) => {
+      for (const mutation of mutations) {
+        if (mutation.type === 'attributes' && mutation.attributeName === 'data-theme') {
+          onAppThemeChange(getEffectiveAppTheme());
+        }
+      }
+    });
+
+    themeObserver.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ['data-theme']
+    });
+  }
+
+  /**
+   * Check if setting change needs a markdown document re-render.
+   */
+  function needsRerender(key) {
+    if (key === 'hljsTheme') return false;
+    return true;
+  }
+
+  /**
+   * Generic apply method for any setting key.
+   */
+  async function apply(key, value) {
+    if (key === 'hljsTheme') {
+      return applyHighlightTheme(value);
+    } else if (key === 'mathFont') {
+      return applyMathFont(value);
+    } else if (key === 'mermaidTheme') {
+      return applyMermaid({ theme: value, look: _activeMermaidLook });
+    } else if (key === 'mermaidLook') {
+      return applyMermaid({ theme: _activeMermaidTheme, look: value });
+    } else if (key === 'emojiSkinTone') {
+      return applyEmojiSkinTone(value);
+    }
+  }
+
+  /**
+   * Apply all settings simultaneously.
+   */
+  async function applyAll(settings) {
+    if (!settings) return;
+    const tasks = [];
+    if (typeof settings.hljsTheme !== 'undefined') {
+      tasks.push(applyHighlightTheme(settings.hljsTheme));
+    }
+    if (typeof settings.mathFont !== 'undefined') {
+      tasks.push(applyMathFont(settings.mathFont));
+    }
+    if (typeof settings.mermaidTheme !== 'undefined' || typeof settings.mermaidLook !== 'undefined') {
+      tasks.push(applyMermaid({
+        theme: settings.mermaidTheme !== undefined ? settings.mermaidTheme : _activeMermaidTheme,
+        look: settings.mermaidLook !== undefined ? settings.mermaidLook : _activeMermaidLook
+      }));
+    }
+    if (typeof settings.emojiSkinTone !== 'undefined') {
+      tasks.push(applyEmojiSkinTone(settings.emojiSkinTone));
+    }
+
+    await Promise.all(tasks);
+  }
+
+  // Public API exported to window.visualSettingsAdapters
+  const adapters = {
+    getOptions: () => JSON.parse(JSON.stringify(OPTIONS)),
+    getDefaults: () => Object.assign({}, DEFAULTS),
+    getCurrentSettings: () => ({
+      hljsTheme: _activeHljsTheme,
+      mathFont: _activeMathFont,
+      mermaidTheme: _activeMermaidTheme,
+      mermaidLook: _activeMermaidLook,
+      emojiSkinTone: _activeEmojiSkinTone
+    }),
+    getCurrentMathFont: () => _activeMathFont,
+    getMermaidConfig: (themeOverride) => ({
+      theme: resolveMermaidThemeName(_activeMermaidTheme, themeOverride),
+      look: _activeMermaidLook === 'handDrawn' ? 'handDrawn' : 'classic'
+    }),
+    getHljsTheme: (themeOverride) => resolveHljsThemeName(_activeHljsTheme, themeOverride),
+    resolveHljsThemeName,
+    needsRerender,
+    apply,
+    getCanonicalEmojiShortcode: (code) => getCanonicalEmojiShortcode(code, typeof joypixels !== 'undefined' ? joypixels : (typeof window !== 'undefined' ? window.joypixels : undefined)),
+    resolveEmojiShortcode,
+    applyHighlightTheme,
+    applyMathFont,
+    applyMermaid,
+    applyEmojiSkinTone,
+    applyAll,
+    onAppThemeChange
+  };
+
+  if (typeof window !== 'undefined') {
+    window.visualSettingsAdapters = adapters;
+  }
+  if (typeof module !== 'undefined' && module.exports) {
+    module.exports = adapters;
+  }
+
+})();
diff --git a/visual-settings-store.js b/visual-settings-store.js
new file mode 100644
index 00000000..a73167be
--- /dev/null
+++ b/visual-settings-store.js
@@ -0,0 +1,183 @@
+(function () {
+  'use strict';
+
+  const STORAGE_KEY = 'mdv.visualSettings.v1';
+
+  const DEFAULT_SETTINGS = Object.freeze({
+    hljsTheme: 'auto',
+    mathFont: 'mathjax-modern',
+    mermaidTheme: 'auto',
+    mermaidLook: 'classic',
+    emojiSkinTone: 'default'
+  });
+
+  const DEFAULT_OPTIONS = Object.freeze({
+    hljsTheme: Object.freeze([
+      'auto', 'github', 'github-dark', 'monokai', 'dracula',
+      'atom-one-dark', 'atom-one-light', 'solarized-dark', 'solarized-light',
+      'vs', 'vs2015', 'nord', 'tokyo-night-dark', 'tokyo-night-light', 'default'
+    ]),
+    mathFont: Object.freeze([
+      'mathjax-modern', 'mathjax-tex', 'mathjax-stix2', 'mathjax-asana',
+      'mathjax-bonum', 'mathjax-dejavu', 'mathjax-fira', 'mathjax-pagella',
+      'mathjax-schola', 'mathjax-termes'
+    ]),
+    mermaidTheme: Object.freeze([
+      'auto', 'default', 'neutral', 'dark', 'forest', 'base'
+    ]),
+    mermaidLook: Object.freeze([
+      'classic', 'handDrawn'
+    ]),
+    emojiSkinTone: Object.freeze([
+      'default', 'tone1', 'tone2', 'tone3', 'tone4', 'tone5'
+    ])
+  });
+
+  function getEffectiveDefaults() {
+    if (typeof window !== 'undefined' && window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getDefaults === 'function') {
+      try {
+        const defs = window.visualSettingsAdapters.getDefaults();
+        if (defs && typeof defs === 'object') {
+          return Object.assign({}, DEFAULT_SETTINGS, defs);
+        }
+      } catch (e) {
+        console.warn('Error reading defaults from visualSettingsAdapters:', e);
+      }
+    }
+    return Object.assign({}, DEFAULT_SETTINGS);
+  }
+
+  function getAllowedValues(key) {
+    if (typeof window !== 'undefined' && window.visualSettingsAdapters && typeof window.visualSettingsAdapters.getOptions === 'function') {
+      try {
+        const opts = window.visualSettingsAdapters.getOptions();
+        if (opts && Array.isArray(opts[key])) {
+          return opts[key].map(item => (typeof item === 'object' && item !== null ? item.value : item));
+        }
+      } catch (e) {
+        console.warn('Error reading options from visualSettingsAdapters:', e);
+      }
+    }
+    return DEFAULT_OPTIONS[key] || [];
+  }
+
+  function validateSettings(settings) {
+    const defaults = getEffectiveDefaults();
+    const valid = {};
+    for (const key of Object.keys(DEFAULT_SETTINGS)) {
+      const allowed = getAllowedValues(key);
+      const val = settings && settings[key];
+      if (typeof val === 'string' && allowed.includes(val)) {
+        valid[key] = val;
+      } else {
+        valid[key] = defaults[key];
+      }
+    }
+    return valid;
+  }
+
+  function loadFromStorage() {
+    const defaults = getEffectiveDefaults();
+    try {
+      if (typeof localStorage === 'undefined') return defaults;
+      const raw = localStorage.getItem(STORAGE_KEY);
+      if (!raw) return defaults;
+      const parsed = JSON.parse(raw);
+      return validateSettings(parsed);
+    } catch (e) {
+      console.warn('Failed to parse visual settings from localStorage:', e);
+      return defaults;
+    }
+  }
+
+  function saveToStorage(settings) {
+    try {
+      if (typeof localStorage !== 'undefined') {
+        localStorage.setItem(STORAGE_KEY, JSON.stringify(settings));
+      }
+    } catch (e) {
+      console.warn('Failed to save visual settings to localStorage:', e);
+    }
+  }
+
+  let currentSettings = loadFromStorage();
+  const listeners = new Set();
+
+  function notify(changedKey, changedValue) {
+    const settingsSnapshot = getSettings();
+    listeners.forEach(cb => {
+      try {
+        cb(settingsSnapshot, changedKey, changedValue);
+      } catch (e) {
+        console.error('Error in visual settings change listener:', e);
+      }
+    });
+  }
+
+  function getSettings() {
+    return Object.assign({}, currentSettings);
+  }
+
+  function updateSetting(key, value) {
+    if (!Object.prototype.hasOwnProperty.call(DEFAULT_SETTINGS, key)) {
+      console.warn('Unknown visual setting key:', key);
+      return false;
+    }
+    const allowed = getAllowedValues(key);
+    if (!allowed.includes(value)) {
+      console.warn(`Invalid value "${value}" for visual setting "${key}"`);
+      return false;
+    }
+    if (currentSettings[key] === value) {
+      return true;
+    }
+    currentSettings[key] = value;
+    saveToStorage(currentSettings);
+    notify(key, value);
+    return true;
+  }
+
+  function resetSettings() {
+    const defaults = getEffectiveDefaults();
+    currentSettings = Object.assign({}, defaults);
+    saveToStorage(currentSettings);
+    notify('*', null);
+    return getSettings();
+  }
+
+  function onSettingsChange(cb) {
+    if (typeof cb === 'function') {
+      listeners.add(cb);
+    }
+    return function unsubscribe() {
+      listeners.delete(cb);
+    };
+  }
+
+  // Refresh settings when adapters become available or options change
+  function refreshValidation() {
+    currentSettings = validateSettings(currentSettings);
+    saveToStorage(currentSettings);
+  }
+
+  const store = {
+    STORAGE_KEY,
+    DEFAULT_SETTINGS,
+    DEFAULT_OPTIONS,
+    getSettings,
+    updateSetting,
+    resetSettings,
+    onSettingsChange,
+    validateSettings,
+    getAllowedValues,
+    refreshValidation
+  };
+
+  if (typeof window !== 'undefined') {
+    window.visualSettingsStore = store;
+  }
+
+  if (typeof module !== 'undefined' && module.exports) {
+    module.exports = store;
+  }
+})();