[defanor]

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.

[alexanderby]

Dark Reader also offers static CSS support https://darkreader.org/blog/stylish/

Global CSS will work well on text websites. For many others it will break up the coloring and make page parts hardly distinguishable.

[kristofferR]

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.

[alexanderby]

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.