Change route-label hover background colour to work with dark theme

Hard-coding a light background for route-label hovering didn't work
well with dark themes which have text in a light colour. Use
@theme_selected_bg_color as suggested by @szszoke.

Fixes #3.

src/alsa-scarlett-gui.css

@@ -3,12 +3,12 @@
 }
 
 .route-label:hover {
-  background: #e0e0e0;
+  background: @theme_selected_bg_color;
 }
 
 .route-label:drop(active) {
   box-shadow: none;
-  background: #e0e0e0;
+  background: @theme_selected_bg_color;
 }
 
 button {