This userstyle is aimed to theme most sites you visit with dark colors. It doesn’t work flawlessly on every site, nor does it intend to do so (it’s impossible to make a style look good in every page with general rules), the main purpose is to just darken most sites you don’t frequently visit (and thus finding specific userstyles for each one would be cumbersome apart from very inefficient) with an all-around dark style. This same principle applies to all global styles.
It also paints scrollbars dark on all major browsers (Chrome, Firefox, and most Chromium-based browsers)
With this in mind, this style won’t theme sites like Google, Reddit or Youtube by default (unless you remove their exclusions from the regexp in the style), since they either have a dark mode, or have better userstyles specifically designed for them out there.
You can find how to add exclusions to the global style here.
A preview of options you can set in your extension