feat: add render.Options.theme to write the theme as literal values

.changeset/theme-option.md

@@ -0,0 +1,5 @@
+---
+'ondine': patch
+---
+
+Added the `theme` render option, which writes the theme into the SVG as literal values. The output has no `var()` or `color-mix()`, so renderers without CSS custom properties draw it, such as resvg, librsvg, and native SVG parsers. Named, `rgb()`, `rgba()`, hex, and `transparent` colors mix. Only `ondine/compiler` and the `all.fast` module support the option.

crates/compiler/Cargo.toml

@@ -9,7 +9,7 @@ rust-version.workspace = true
 version.workspace = true
 
 [features]
-default = ["all"]
+default = ["all", "theme"]
 # Every diagram type, for native builds and tests.
 all = [
   "agentflow",
@@ -82,6 +82,9 @@ wardley = []
 xychart = []
 # Serializes parsed models as JSON for parity tests. Release modules leave it out.
 model = []
+# Writes the `theme` render option into the SVG as literal values. Only the speed-optimized `all`
+# module includes it, so the size-optimized browser modules leave out the resolve pass.
+theme = []
 
 [dev-dependencies]
 insta.workspace = true

crates/compiler/src/error.rs

@@ -9,6 +9,8 @@ pub enum Code {
     Semantic,
     /// The source does not match the diagram's grammar.
     Syntax,
+    /// The options ask for something that this compiler module does not support.
+    Unsupported,
     /// No supported diagram type was detected.
     UnsupportedDiagram,
 }
@@ -20,6 +22,7 @@ impl Code {
             Self::Limit => "limit",
             Self::Semantic => "semantic",
             Self::Syntax => "syntax",
+            Self::Unsupported => "unsupported",
             Self::UnsupportedDiagram => "unsupported-diagram",
         }
     }

crates/compiler/src/lib.rs

@@ -24,6 +24,8 @@ mod svg {
     pub(crate) mod builder;
     pub(crate) mod cluster;
     pub(crate) mod edges;
+    #[cfg(feature = "theme")]
+    pub(crate) mod resolve;
     pub(crate) mod root;
     pub(crate) mod shapes;
     pub(crate) mod theme;
@@ -171,6 +173,13 @@ pub fn render(source: &str, options: &str, now: f64) -> Result<Rendered, Error>
     };
 
     let drawn = (diagram.render)(&input, &options, &url)?;
+    #[cfg(feature = "theme")]
+    let document = match &options.theme {
+        Some(theme) => svg::resolve::resolve(&drawn.svg, theme)?,
+        None => drawn.svg,
+    };
+    #[cfg(not(feature = "theme"))]
+    let document = drawn.svg;
     let mut rendered = Rendered {
         height: drawn.height,
         links: drawn
@@ -182,7 +191,7 @@ pub fn render(source: &str, options: &str, now: f64) -> Result<Rendered, Error>
                 internal: link.internal,
             })
             .collect(),
-        svg: drawn.svg,
+        svg: document,
         r#type,
         width: drawn.width,
     };
@@ -287,6 +296,15 @@ fn options_from_json(json: &str) -> Result<svg::root::RenderOptions, Error> {
             "outputLimit" => options.output_limit = number(value),
             "padding" => options.padding = number(value),
             "responsive" => options.responsive = boolean(value),
+            #[cfg(feature = "theme")]
+            "theme" => options.theme = svg::resolve::properties(value),
+            #[cfg(not(feature = "theme"))]
+            "theme" => {
+                return Err(Error::new(
+                    Code::Unsupported,
+                    "This module does not support the theme option. Use ondine/compiler.",
+                ));
+            }
             _ => {}
         }
     }