Phew, GitHub is over ten years old now... and is unquestionably synonomous with open source. After 10 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... on hover for desktop, and on page load for mobile devices.
You can build your own with the SVGs, copy some of the pre-defined code below, or just change the fill and color values on the main <svg> element.