May I ask (genuine question), why would anyone need a choice per site? Surely some people are “light mode” people and want everything in that mode and others are “dark mode” people?
Some people may not want everything to be light or dark mode. I am one of them.
Some articles may not work well with dark mode.
Like someone above said, some sites implement it as an after-thought. In my case, I'm giving more love to dark-more because there are simply more nice color combinations in dark more than with white.