If you’re looking for a browser extension like Stylus (which lets you override browser styles, scoped to specific sites) for Safari on macOS, check out Cascadea. It’s by an independent developer (Mike Freuden), doesn’t farm you for your data, and seems to do what it says on the tin (in my limited few minutes of testing right now¹) :)
¹ I’m using it to hide the controls on the local export of my slides.com presentation.