buttons: fixup colors and text/icon-shadow for toolbar buttons
buttons: fixup colors and text/icon-shadow for toolbar buttons

Fixup some regressions in color and text-icon shadow for the backdrop
state.

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