I've been using Stylus for many months now, it works exactly the same but is open source.

That, combined with "Dark Background + Light Text" with custom colors makes Firefox a beautiful sight in my personal night mode.

"Dark Background" for the majority of sites that don't have good dark Stylus themes, disabled on the ones that do.

This one is nice:

https://github.com/dparpyani/HackerNews-DarkTheme