Fix class-based dark theme variant

Agent-Logs-Url: https://github.com/Hack23/cia-compliance-manager/sessions/8afb7db1-569b-4501-9612-e7adf0fb4c2d

Co-authored-by: pethers <1726836+pethers@users.noreply.github.com>

Author: Copilot

src/index.css

@@ -12,6 +12,9 @@
 @import "./styles/light-theme.css";
 @import "./styles/dark-theme.css";
 
+/* Use the app-controlled `.dark` class instead of OS color-scheme media queries. */
+@custom-variant dark (&:where(.dark, .dark *));
+
 /* Global styles that use CSS variables */
 :root {
   /* Any global variables should be here, not outside a selector */