Fix theme export behavior by cleaning up DOM elements

Ensure the temporary link element is removed after triggering the export action to prevent cluttering the DOM. Additionally, display a success message upon successful theme export for better user feedback.

src/browser/components/preferences/zen-settings.js

@@ -242,6 +242,8 @@ var gZenMarketplaceManager = {
 
       document.body.appendChild(a);
       a.click();
+      a.remove();
+      successBox.hidden = false;
     } catch (error) {
       console.error('[ZenThemeMarketplaceParent:settings]: Error while exporting themes:', error);
       errorBox.hidden = false;