Open up about:config and go to layout.css.prefers-color-scheme.content-override and set it to one of the following:
0 => Force a Dark background
1 => Force a Light background
2 => Follow the System theme (default in Firefox 94)
3 => Follow the Browser toolbar theme (default in Firefox 95+)
that does not force the websites, but tells them to show the dark style if they have one. It’s activated if your browser is set to a dark theme
I know and I don’t like it. I want a dark browser theme and normal websites
fair enough. but I think I remember an option on the main settings page that allows you to change it this way