gtk3: Provide a background-color for menubars
gtk3: Provide a background-color for menubars

https://bugzilla.gnome.org/show_bug.cgi?id=730016

Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard