Isn't the actually-simple answer here "Pick a default. If the user agent specifies a preference, respect it."

Now there's no UX, it just does something reasonable regardless, and if the user has already opted in to color mode management, it respects their declared choice.

Users who are power user enough to want to override their light/dark mode setting per-site can use a browser extension to let them do exactly that on the websites they want to target.

Am I missing something?