So this took me about an hour to diagnose. Most of the documentation for adding a Drupal 8 library (groupings of CSS and JS) to a custom (thing) is based on a custom theme or child theme. So, they say you do the following in your mytheme.info.yml:
Like many web developers, I've been using CSS frameworks for years - Bootstrap and Foundation being the main ones. Of course using these has been incredibly helpful when it came to layout and not having to worry about mobile responsiveness, layout percentages, mobile menu's built in, and some other extras like accordions and such. While I love these frameworks, I kept hearing about CSS Gris as a replacement and now that browser support has gotten really good, I thought why not give it a try?