Firefox

Easy. Preferences > Contrast control > Manage colors and set the background color to black or whatever.

You will never be whiteflashed again. Some websites will be ugly, a few will be unusable (you'll have to unset it temporarily for them, or use a different profile). But many will be just as good as with Dark Reader.

In any case, this is literally the only way to banish whiteflash definitively. It also has no speed penalty because it's native.