chore: Update Zen theme CSS to use CSS variables for element separation

src/browser/base/content/ZenStartup.mjs

@@ -27,6 +27,7 @@ var ZenStartup = {
     document.getElementById("zen-appcontent-navbar-container")
       .appendChild(document.getElementById("tab-notification-deck-template"));
 
+    gZenThemeMarketplace.init();
     gZenVerticalTabsManager.init();
     gZenCompactModeManager.init();
     gZenKeyboardShortcuts.init();