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.
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.
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.