GitHub Desktop has its own set of illustrations, which were adopted from a previous GitHub Universe with a space theme. Illustrations primarily show up in the welcome wizard and empty states for lists areas. Because Desktop has a dark mode, hard-coded colors need to look good on either white or dark gray backgrounds. They are composed of 1 or 2 swatches that get swapped out with a css invert filter on. If a shadow is included in the illustration, it needs to be translucent.
All of the illustrations can be found in the desktop repo.