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.
how your users' browsers choose to render `about:blank` while waiting on your page to be delivered is outside of both your control and concern
on Gnome i've got system-wide dark mode turned on and idk, my Firefox is dark gray until it gets any content. so users have the power and should exercise it to tailor their experience as they wish
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.