This repository serves as a home for all the user styles I've written for the stylish plugin on firefox (though they may work with other browsers as well).
Note: there is a stylish extension for chrome as well.
First you'll need to install stylish, then you can open the css file you want to install in firefox (Example: github_dark.css), click the stylish icon in the status bar, choose "Install file...", and then follow stylish's directions from there.