GPL userstyle Firefox add-on. But wait, there’s more! It is a response to some bullshit Stylish pulled:
On the 2nd of July, 2018, it was discovered that the extension was tracking its users’ browsing history and sending them to a remote server operated by its parent company, SimilarWeb. It was subsequently removed from the official Firefox, Chrome, Opera and Safari extension websites and is no longer available to install.
Awesome! And I don’t even use userstyles!