Merely go here and install using stylish, if you want to create your own read on...
sass github-thalhalla.scss github-thalhalla.css
Then copy the resulting github-thalhalla.css into stylish and associate with your selected slack domain.
make
Using the make method will automatically copy the resulting github-thalhalla.css to your clipboard (assuming you are in X). Then just paste into stylish
You can just copy the contents of github-thalhalla.css into a stylish document.