fix: register themes on extension load not start (#22148)

packages/cli/src/config/extension-manager.ts

@@ -564,7 +564,7 @@ Would you like to attempt to install via "git clone" instead?`,
 
   protected override async startExtension(extension: GeminiCLIExtension) {
     await super.startExtension(extension);
-    if (extension.themes) {
+    if (extension.themes && !themeManager.hasExtensionThemes(extension.name)) {
       themeManager.registerExtensionThemes(extension.name, extension.themes);
     }
   }
@@ -624,6 +624,13 @@ Would you like to attempt to install via "git clone" instead?`,
 
         this.loadedExtensions = builtExtensions;
 
+        // Register extension themes early so they're available at startup.
+        for (const ext of this.loadedExtensions) {
+          if (ext.isActive && ext.themes) {
+            themeManager.registerExtensionThemes(ext.name, ext.themes);
+          }
+        }
+
         await Promise.all(
           this.loadedExtensions.map((ext) => this.maybeStartExtension(ext)),
         );