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.

Prerequisite
Override Styles
Override Settings
Override Templates and Theme Functions

Prerequisite

Read the @link subtheme Sub-theming @endlink parent topic.

Override Styles {#styles}

Open ./THEMENAME/css/style.css and modify the file to your liking.

Override Settings {#settings}

Please refer to the @link theme_settings Sub-theme Settings @endlink topic.

Override Templates and Theme Functions {#registry}

Please refer to the @link registry Theme Registry @endlink topic.