fix: theme mode not correctly applied if selected manually

frontend/src/app.css

@@ -1,6 +1,8 @@
 @import 'tailwindcss';
 @import 'tw-animate-css';
 
+@variant dark (&:where(.dark, .dark *));
+
 /*
   The default border color has changed to `currentcolor` in Tailwind CSS v4,
   so we've added these compatibility styles to make sure everything still