@smsmith Did. Now the settings have become black, but before that, a white background opens for half a second. Also the left menu has disappeared.
Yeah, that's how the Material Design settings will work, no menu, you'll just scroll down through the settings. It's a tradeoff.
The white flash is always going to happen, since the white background is the blank window before loading a page. Opera would need to change that for the dark theme. I understand your frustrations though. If you're going for all dark and then suddenly have a blinding flash of white, even for as short as it is, it can be pretty jarring. In the meantime, maybe just close your eyes when you click to open the settings page?