Update for GTK+ master changes
Update for GTK+ master changes

GTK+ master now renders background-color below background-image, so we
need to set a transparent background color when we don't want it rendered.

magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard