titlebar: set a default background color for backdrop titlebars
titlebar: set a default background color for backdrop titlebars

https://bugzilla.gnome.org/show_bug.cgi?id=706068

rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark