The add-on this uses steals your browser history.
https://www.bleepingcomputer.com/news/software/chrome-and-firefox-pull-stylish-add-on-after-report-it-logged-browser-history/
I have removed the links to the add-on and I highly recommend anyone using this add-on to remove it from their browser. Just to elaborate, this has NOTHING to do with what Hova has done. He made a theme that this particular add-on uses to change the looks of the page. The add-on developer had malicious intentions.
I would recommend that @Hova use something like Stylus instead if you want to continue using a dark theme.