Currently, to toggle on dark mode in Windows 11, you have to go to Settings > Personalization > Colors, and then change the "Choose your mode" setting to dark to enable it. So, at least with it being ...
Now, just like Android or iOS, you'll be able to change themes on Windows 11 from Quick Settings. Rolling out soon.
Annoying white flashes are still hanging around in Windows 11 for some users, but hopefully not for much longer.
Two clicks and you're done.
Energy-saving mode was originally intended (primarily) for laptops. With a recent Windows update, power consumption can now also be permanently reduced on desktop PCs.