Overlay icons perfectly visible against the background color in standard
and full screen mode. Tested with "Breeze" and "Breeze Dark" colour
schemes as well as "Breeze", "Oxygen", "Fusion" and "GTK2/Adwaita"
widget styles. (It breaks with "GTK2/Breeze", but that's nothing for
Gwenview to fix. If you want "Breeze", run the native one.)