Tiny refactoring

- use checkradio_fg_color for check/radio in treeview, which points to fg_color, so no change of the hex color, thus no change to the parsed CSS

gtk/theme/Adwaita/_common.scss

@@ -2947,7 +2947,7 @@ treeview.view check,
 treeview.view radio {
   &:selected {
     &:focus, & {
-      color: $fg_color;
+      color: $checkradio_fg_color;
 
       @if $variant == 'light' { border-color: $selected_borders_color; }
     }