At least for Firefox, having the browser chrome set to dark (typically by having it picked up from GTK theme preferences) should make the default page backgroun

At least for Firefox, having the browser chrome set to dark (typically by having it picked up from GTK theme preferences) should make the default page background dark too. And I think userchrome.css to override default bgcolor should still work. Of course there are individual wonky sites which will still flash but if applicable, those two things should reduce the occurrences significantly.