[roryokane]

Though Stylish was compromised, you can safely use the fork Stylus that was created in response: https://github.com/openstyles/stylus

I use Stylus to add custom CSS to sites and Violentmonkey (https://violentmonkey.github.io/) to add custom JS. They both make it fairly easy to start writing code for a new site. However, there is no easy way to set up both custom CSS and custom JS for a single site – a custom browser extension like you made could potentially support that better.