fix(theme-constroller): fix theme flashing behavior when switching custom themes in apply() (@raaid3) (monkeytypegame#6670)

### Description

Closes [monkeytypegame#6669](monkeytypegame#6669)

<!-- the issue(s) your PR resolves if any (delete if that is not the
case) -->
<!-- please also reference any issues and or PRs related to your pull
request -->
<!-- Also remove it if you are not following any issues. -->

<!-- pro tip: you can mention an issue, PR, or discussion on GitHub by
referencing its hash number e.g:
[monkeytypegame#1234](monkeytypegame#1234) -->

<!-- pro tip: you can press . (dot or period) in the code tab of any
GitHub repo to get access to GitHub's VS Code web editor Enjoy! :) -->

Author: raaid3

frontend/src/ts/controllers/theme-controller.ts

@@ -153,7 +153,9 @@ async function apply(
     customColorsOverride,
     isPreview
   );
-  clearCustomTheme();
+  if (!Config.customTheme) {
+    clearCustomTheme();
+  }
   const name = customColorsOverride ? "custom" : themeName;
 
   ThemeColors.reset();