W32: Fix dummy prelight style for check/radio buttons

Use a dummy transparent gradient instead of actual color (which screws up
focus rectangle on prelight), and apply to non-prelight state as well.

https://bugzilla.gnome.org/show_bug.cgi?id=728807

gtk/resources/theme/gtk-win32-base.css

@@ -160,9 +160,11 @@ GtkRadioButton:prelight:selected {
 }
 
 /* This is meaningless, it just enables prelight propagation to check/radio mark */
+GtkCheckButton,
+GtkRadioButton,
 GtkCheckButton:prelight,
 GtkRadioButton:prelight {
-  color: @fg_color;
+  background-image: -gtk-gradient (linear, left top, right bottom, from(transparent), to(transparent));
 }
 
 .check {