For GH you can set your theme preference in your account settings.

I've had it on a for a while and I don't seem to have any issues getting dark theming to stick.

You could always step out of the cave and join the rest of humanity in the light.

I guess you could set up custom stylesheets, at least for sites you commonly browse.

The time-based light/dark mode setting requires the current time. It is not currently accessible from CSS and is blocked by resistFingerprinting

Does that for extensions like Dark Reader Pro too?

Dark Reader extension has made me never notice this was a thing.