Phew, Github is over seven years old now... and is unquestionably synonymous with open source. After 7 years, those GitHub ribbons are more than overdue for a cleaner, more modern alternative. This is my take.
By using SVG, these corners can be clean and sharp. Any color, and size. As well as have a little character and interactivity!
You can build your own with the SVG's, or copy some of the pre-defined code.
You can see the page here ... but I really recommend visiting the page to see the small animations that really make them come to life.
Naturally, these aren't built by GitHub, or endorsed by them.
Copyright (c) 2015 Tim Holman - http://tholman.com
The MIT License (MIT)