The CDN Starterkit is rather simple to set up. You don't have to do anything until you wish to override the default Drupal Bootstrap base theme settings or provide additional custom CSS.
Read the @link subtheme Sub-theming @endlink parent topic.
Open ./subtheme/css/style.css and modify the file to your liking.
Please refer to the @link subtheme_settings Sub-theme Settings @endlink topic.
Please refer to the @link registry Theme Registry @endlink topic.