Stylish Replacement for Mac

Solution 1:

There is a paid ($3 in US) application Cascadea. I’m happy with it, because it is Safari App Extension instead of the Legacy Safari Extensions (.safariextz files). It seems to only work with User CSS (*.user.css) though, so you may want to write to support team ([email protected]) to confirm before you pay.

Solution 2:

I previously used dotjs and dotcss for Safari. As I relied on these a lot, I've written an extension for Safari 12 that provides the same functionality in one extension with a simpler setup.

Not sure if this is enough for your needs as it only offers injection of CSS and JS files based on the page's domain name.

In case you want to find out more, I've written an introductory blog post. The extension is available on the App Store ($5). It's open source, so you're also welcome to compile it yourself for free.