gtk3: Theme spinner inside GdMainIconView's cells
gtk3: Theme spinner inside GdMainIconView's cells

Make spinner visible on both dark and bright backgrounds w/o making
it look ugly/weird.

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