David Kimura PRO said almost 4 years ago on Multitenancy :
  Look under the app/assets/drifting_ruby.css. It is mainly taken from Bootstrap 4, but I only picked and chose a few most commonly used items as well as adding in a few of my own. The idea was to have as "fresh" of an application as possible so that it didn't interfere with the episode content. A lot of videos out there will use a template which has lots of preconfigured stuff and it makes it difficult to see what is episode content vs template content. So beyond the stylesheet, there isn't anything else.