While it is nice, there are more lightweight and snappy ways to achieve a dark background (or desirable styles in general) -- such as setting a global CSS, using built-in color overriding (at least in FF it's available, along with font overriding), using web browsers that don't apply CSS. Some people even use system-wide color inversion. Each method has its pros and cons though.
Stylus is one of the handy FF extensions, which allows to switch between CSS themes quickly/easily.
Any plans to implement proper UserStyles.org support where themes can auto-update like with Stylus? Some of the most popular style sheets update quite frequently.
Stylus already does it well, so I think you could use Stylus for such websites. UserStyles.org is still owned by SimilarWeb, so I would stay away from it. What is planned is to add some static themes for popular websites.
Stylus is one of the handy FF extensions, which allows to switch between CSS themes quickly/easily.