This is basically a dead project. And BTW, stay away from stylish and sylus (same company). They are nothing but spyware now and will ruin FF performance. Trust me. Or don't. It's your time you'll waste.

Nice.

Pleasantly simple (user-friendly) compared to alternatives such as Stylus.

Does work with styles at https://userstyles.org/ – the amber 'Add to Website Theme Manager' button, part-way down the page for a style.