EDIT These rules do not work with the current desktop-browser design for Twitter (mid-2019). Try uBlock Origin with these rules instead.
But if you're somehow using an old Twitter design…
In Safari's preferences, the ability to choose a custom CSS file is under the Advanced tab. As far as I can tell, you must quit and re-open Safari when you make changes to the file to see those changes take place on the sites you're browsing.