The addon is great to change the appearance of a website, however, it seems it canNOT be used with VK. If either Stylus or Stylish (it doesn't matter, I tried) is on, VK page elements, be it the messenger or literally any other page, either move up and away from the view, or the browser shows the page with no CSS or HTML formatting active, just plain text. This only happens on Firefox, too, on all Chromium-based browsers, VK works fine with both Stylish and Stylus. It also doesn't matter if a VK-specific style is active or not, the pages break even with no styles active. This addon breaks only VK pages, all other websites work as they should.

Great add-on. userstyles.org website appears to be stable now.

Perfect! I needed something to hide the cookie policy dialog on amazon.it and it took me just one minute to do it using this web-ext.