This theme will make every site dark(er).
It’s impossible to make sure every website will stay usable, but it should help with eye-strain.
This theme is a last-effort, ideally you should look for a theme that’s made for the site you want to use. You can also request a dark theme from the community here: https://forums.openusercss.org/category/7/theme-requests
A preview of options you can set in your extension