Bug in add-on when using a private window.
Steps to reproduce:
In a browser with app not installed...
Open a new private window
Install the app from the add-on manager or userstyles website
Install theme from userstyles.org
Theme will not load when visiting designated site

Recomend use Stylus add-on instead

The idea is great, but the latest version uses HEAPS of CPU. The deployed version of the app doesn't match the one in the repository and there is no transparency despite a long history of privacy abuses. This whole project is an essay in the slow decay of communal resources, why we cannot have nice things etc.

This addon is garbage. Use Stylus instead. Unfortunately you will still have to deal with userstyles.org (the ones behind this extension) and that piss poor website.

See why it's called spyware: https://en.wikipedia.org/wiki/Stylish#Privacy_issues

Why is Mozilla the self proclaimed Privacy Advicate and Savior of the Web even allow spyware on its extension store? But when I review APB and its shady practices and history Mozilla allows the developers to censor AS IN REMOVE the review.

This addon isn't what it used to be. Now the only thing it does is not working! I installed 2 dark themes for Yandex and not only none of them appeared as "installed" in the addon (next to "library") but they also didn't have any effect on the yandex page.
Deleting from the browser, it's not worthy to waste my time with this crap.

Spyware; use Stylus instead.