As other's have said, the Godot docs do not have a dark theme. I'd assume you must be running a theme library of sorts in your browser? For example, I use
stylish for some sites (though, not for
godotengine.org). However, I see that I can install a dark theme for the Godot docs via the
stylish theme library. Are you sure you aren't already using such a library? If you are, simply disable it for the godot docs site.