Fix dark mode for Tailwind CSS v4

- Add @custom-variant for class-based dark mode in CSS
- Tailwind v4 no longer uses tailwind.config.js darkMode setting with @import syntax

Author: Hilichiu

src/index.css

@@ -1,5 +1,8 @@
 @import "tailwindcss";
 
+/* Enable class-based dark mode for Tailwind v4 */
+@custom-variant dark (&:where(.dark, .dark *));
+
 /* Custom slider styling */
 .slider {
   -webkit-appearance: none;