Cornerstone is built into all Kajabi Starter themes. Cornerstone was built with you the developer in mind. We find that when building Kajabi Themes there are a number of things that we always do to get our CSS styles up and running. This takes those items and builds them in so you can get directly to creating your theme specific style. We also built this with the idea that if you choose to use Boostrap4, the class names will line up so your markup will not need to be changed. Just swap it out.