themes: Keep headerbar color consistent when out of focus

On some system themes, the background color of
menu items on a merged headerbar would be a
different color from the rest of the menu when
GIMP's main window was out of focus.
This patch defines those items as clear so they become
whatever color the rest of the headerbar is instead.

themes/Default/common.css

@@ -756,6 +756,10 @@ headerbar:backdrop * {
   color: @disabled-fg-color;
 }
 
+headerbar:backdrop menuitem {
+  background-color: transparent;
+}
+
 /* Prevent overlapping menu/header borderlines for CSD */
 headerbar button.titlebutton, headerbar > menubar {
   box-shadow: none;