I agree with user Chris, below, one star only because of the flash of white.

Also, to "Firefox user 4981570" who suggests updating/creating userchrome.css, your suggestion doesn't remove the white flash.

And, finally, to "Developer response" below, your suggestion doesn't work on Firefox for Android. Did you forget that this browser runs on multiple platforms, not just the one you work on?

In summary, total garbage. As per.

there is a delay from white to black

A black page is useless. Black background with actual buttons, now that's useful.