I often use different light/dark settings between apps and my system. Just because I want system UIs to be dark, for example, doesn't mean I want to read long pages of white-on-black prose on your blog.

Then you still don't need a switch on every website. Just set the browser to display the light version and have it ignore the system setting.

That's what we need, for browsers to have a setting to remember our light/dark preferences per-domain.

Seconded. Just because I like to have the browser toolbar dark and GitHub dark doesn't mean I also want to read lengthy articles (LWN) in thin white text on a black background.

My specific usage is a site to host my own internal training content.

I want to read it in dark mode and give users that option, but I want to present it in light mode because dark mode suffers poorly from video compression when screen sharing.

I currently have a JS toggle for it which uses local browser storage, but ditching JS would be nice if possible.

Having it default to the users preference is nice, but you should still provide an override. I sometimes use my browser in light mode while my OS is dark mode. Many times, I find the contrast for dark mode websites too low unless I’m in a totally dark room.

Because I don't want to toggle my whole system theme based on one special website.