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.

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THE BEST