There are some pressed classes defined but I don't think you can dynamically add them when the button is :active through CSS only.
I'm genuinely surprised that this made it to the front page of hacker news. That being said, I still think Nick La is a great designer.
Why do I have to hear about every front-end web dev's effort to reinvent the wheel? Why are users forced to relearn a new visual language to accomplish the exact same task on every website?
You don't have to listen. Forcing people onto something when they'd rather create their own is a much worse proposition.
Took me about 5 mins to upload the github icon and edit the CSS file.