refactor: deduplicate theme-setting script generation

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -335,11 +335,13 @@ public static void applyThemeAttribute(Element element, String theme) {
 
     String script;
     if (element.getTag().equalsIgnoreCase("iframe")) {
-      script = "this.contentWindow.document.documentElement.setAttribute('theme', $0)";
+      script = "let e = this.contentWindow.document.documentElement;";
     } else {
-      script = "document.documentElement.setAttribute('theme', $0);";
+      script = "let e = document.documentElement;";
     }
 
+    script += "e.setAttribute('theme', $0);";
+
     element.executeJs(script, theme);
 
     Component c = element.getComponent().get();