[dzaima]

Your average user wouldn't even know that there's a standard css property that a plugin could potentially control; though, yeah, it being a browser toggle would make it rather obvious (...assuming it's front-and-center somewhere, which is a rather high ask). Definitely won't find a location to place it that requires one click to toggle on mobile browsers, though.

Having the in-browser toggle would also be rather questionable for anything that has more than just "light" and "dark"; the options suck - either having the browser-based toggle be a "shortcut" for one toggle in your main theme settings (which likely means users are less likely to notice the full settings), or abandon the browser toggle entirely (which likely means a significant amount of users would assume there's no theming at all). And neither really fits for something that has, instead of just "dark" and "light", an additional "high-contrast" mode (which, depending on what the site's designers bothered to do, could be an extension of "dark", or a toggle combinable with either light or dark); so you'd want the browser-based toggle to be at least somewhat customizable. Also worth considering is that if there's some fixed set of things that the browser-level theme toggle can do, sites would be very heavily disincentivized from adding any other things as those would then require an entirely custom theming setting panel.

I'm exaggerating somewhat, sure, but I don't think you'll find any way to have a browser-level theme toggle that doesn't end up pretty bad in a significant number of cases.