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.
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 (firstname.lastname@example.org) to confirm before you pay.
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.