I'm not 100% certain that this should be forced by the browser and IMHO it's the websites owners job to provide properly working dark theme and the UI/UE they think fits best their purpose (and not the browsers). Unless the assumption is that styling across all sites should look 100% identical which may appeal to some, though if I really want that then I can go look for content on gemini:// and no page will ever use their own styling other than what I am telling it to. Maybe it's just me but I like being dazzled and surprised with new design and style that I've not seen before.
I agree that all websites should give users a dark mode. They don't, so we have to make our own.
And for $4.99 Safari user. 30% paid to Apple and 70% for developer to recoup their $99/year cost.
Like ad blockers, sometimes you have to adjust it, but it usually Just Works. The main innovation from this is that my display has much improved contrast, because I can set the brightness for text on a black background, instead of setting the brightness for an entire screen of white. It is amazing how good a screen looks when it's not running at 1% brightness. (Just don't open a non dark mode app at night, or you won't be sleeping for a week.)