A small blocking `<script>` in the `<head>` that reads the saved preference from localStorage and sets a class on `<html>` before any rendering happens is the standard approach. You can also set `<meta name="color-scheme" content="dark light">` which tells the browser to use the OS preference for the initial paint, covering the default case without any JS at all.

Use `background-color` in Firefox's `userContent.css`.

I don't know if I misunderstand the problem, but what about a style tag at the earliest part of the page indicating the background color to use?

Came here for this. I hate it too. When I make a site with dark mode, I make sure the dark color loads by default and then changes to light if needed to address this specific issue.

make dark mode the default, then it's a flash of dark in either case

send a blank black page then load from there?

Decrease screen brightness. Turn off dark mode. No flashbang. Bonus: Battery lasts longer.