dark-theme: use a solid color for insensitive background
dark-theme: use a solid color for insensitive background

Or it won't play nice with the way we're sometimes shading the color.
This also fixes buttons having different background colors in the dark
theme depending on where they were placed (as seen in e.g. Totem's
plugins dialog).

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