fix(ui): revert preview theme on dialog unmount (#22542)

Co-authored-by: Jack Wotherspoon <jackwoth@google.com>

packages/cli/src/ui/AppContainer.tsx

@@ -977,6 +977,7 @@ Logging in with Google... Restarting Gemini CLI to continue.
       openAgentConfigDialog,
       openPermissionsDialog,
       quit: (messages: HistoryItem[]) => {
+        closeThemeDialog();
         setQuittingMessages(messages);
         setTimeout(async () => {
           await runExitCleanup();
@@ -1005,6 +1006,7 @@ Logging in with Google... Restarting Gemini CLI to continue.
     [
       setAuthState,
       openThemeDialog,
+      closeThemeDialog,
       openEditorDialog,
       openSettingsDialog,
       openSessionBrowser,