The former is just wrong of OP: no user wants the FOUC flash-of-white, end of story; similarly for some of the other solutions proposed here (block the entire page, really? talk about burning down the village to save it). The latter has more leeway for design: we settled on a tri-toggle to use the media-query but allow users to override and hardwire light/dark mode, and that seems to work most naturally. But there may be other contexts where something else would be better.

Indeed, for toggles, you could even try not having a toggle at all and relying on the system setting through the media-query. (However, as appealing as this is - why have separate controls when the user already has a global control and can enable/disable it? - we found this confused & angered users: https://gwern.net/design-graveyard#automatic-dark-mode So, gotta have a toggle. At least for now; maybe as dark-mode becomes universal, at some point users will become educated enough that you can afford to use only the system setting?)