I'm not sure what you mean
exactly by "I don't want to install a js"?
Does this mean you don't want to run JavaScript at all? Ever? That's a pretty tall order in today's world. I can understand having JavaScript disabled by default. But that said, JavaScript can be useful for what you want.
I know this because I empathize greatly (except I prefer light mode). In any case, when I saw this comment:
https://news.ycombinator.com/item?id=32719488
in a previous thread, I immediately created the appropriate bookmark-let so that my eyes wouldn't go crazy trying to read dark-mode pages.
Maybe it can serve as a solution for you too if you're willing to compromise a bit.