Add color-scheme to dark/light themes for native control styling

Spin buttons, checkboxes, scrollbars, and other native form controls
now render in the correct theme instead of using default browser chrome.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/style.css

@@ -18,6 +18,7 @@
     --text-color: #e0e0e0;
     --border-color: #404040;
     --display-badge-bg: rgba(0, 0, 0, 0.4);
+    color-scheme: dark;
 }
 
 /* Light theme */
@@ -27,6 +28,7 @@
     --text-color: #333333;
     --border-color: #e0e0e0;
     --display-badge-bg: rgba(255, 255, 255, 0.85);
+    color-scheme: light;
 }
 
 /* Default to dark theme */