This doesn't work for Vivaldi for some reason. I get a dark grey screen, text says "Press esc to exit full screen mode" for a few seconds, then that disappears

This doesn't work for Vivaldi for some reason. I get a dark grey screen, text says "Press esc to exit full screen mode" for a few seconds, then that disappears leaving just the dark grey. No matter which colour I select in the app. Seems like a weird thing not to work in a usually pretty good browser but I'm not sure exactly what this is using.