feat: add support for dynamic theme in iframes

Author: javier-godoy

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

@@ -224,10 +224,19 @@ public void apply(HasElement component) {
     assertFeatureInitialized();
 
     VaadinSession.getCurrent().setAttribute(DynamicTheme.class, this);
+
+    String document;
+    if (component.getElement().getTag().equalsIgnoreCase("iframe")) {
+      document = "this.contentWindow.document";
+    } else {
+      document = "document";
+    }
+
     component.getElement().executeJs("""
+        const _document = %s;
         const applyTheme = () => {
           ["lumo/lumo.css", "aura/aura.css"].forEach(href=> {
-            let link = document.querySelector(`link[href='${href}']`);
+            let link = _document.querySelector(`link[href='${href}']`);
             if (!link) return;
             if (href === $0) {
                if (link.rel === 'preload') link.rel = 'stylesheet';
@@ -238,12 +247,12 @@ public void apply(HasElement component) {
           });
         };
 
-        if (document.startViewTransition) {
-          document.startViewTransition(applyTheme);
+        if (_document.startViewTransition) {
+          _document.startViewTransition(applyTheme);
         } else {
           applyTheme();
         }
-        """, href);
+        """.formatted(document), href);
   }
 
 }