theme: Fix vertical spin button selection

The selection should be black-on-blue as it is
everywhere else now. This was just a leftover.

Fixes: #4788

gtk/theme/Default/_common.scss

@@ -1165,7 +1165,7 @@ spinbutton {
         color: transparent;
         &:focus-within {
           background-color: $selected_text_bg_color;
-          color: $selected_fg_color;
+          color: $fg_color;
         }
       }
       > block-cursor { @include entry(block_cursor); }