Update dark mode --background to #09090b to match main branch

Changed only the --background variable from #18181b to #09090b to match
the Termix main branch login page background color.

Changes:
- .dark --background: #18181b → #09090b

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: PeetMcK

src/index.css

@@ -171,7 +171,7 @@
 }
 
 .dark {
-  --background: #18181b;
+  --background: #09090b;
   --foreground: #ffffff;
   --card: #27272a;
   --card-foreground: #fafafa;