If you don't want to pay $2 for Tampermonkey, you can try Userscripts, which is free.
Userscripts is simpler, you will have to add your scripts using the extension page instead of installing them from GreasyFork automatically, for example. But otherwise it does the job of CSS and JS customisations as well. See the documentation of Userscripts for more information.
Mind that you won't be able to install extensions, which tamper with input devices, in recent versions of OSX. After the installation from the App Store you will have to enable them in the safe mode - after restarting holding the
Shift key down. See an article at Apple about it.