I wonder if anyone is aware of any replacement for Stylish. Stylish is not on the Safari Extension Gallery since they were stealing browsing information.
I replaced Stylish with FreeStyler which provides the same functionality just with a slightly worse UI. There’s no automatic import path from files or another extension, so moving styles were a case of manual copy-paste and configuring the applicable hosts/URL prefixes.
I've actually found Tampermonkey. It's not totally Stylish but it can do the same.
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.
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@example.com) to confirm before you pay.