In this article I will show you how to create a Bootstrap subtheme for Drupal 8.
1. First of all - you need to download, install and enable Bootstrap theme (https://www.drupal.org/project/bootstrap).
2. Open the Bootstrap folder and find the folder /starterkits/cdn inside bootstrap theme folder.
3. Copy /cdn folder to your drupal /themes folder so you will have your_site/themes/cdn
4. Rename cdn to your theme name. Lets say it will be /mytheme.
5. Now you need to replace THEMENAME word to mytheme inside your mytheme folder:
6. Starterkit file is needs to be renamed from /THEMENAME.starterkit.yml to /mytheme.info.yml
7. Now open your mytheme/mytheme.info.yml
Change name: 'THEMETITLE' to name: 'mytheme'
9. Open /config/schema/mytheme.schema.yml
Change label: 'THEMETITLE settings' to label: 'mytheme settings'
10. Now go to drupal admin 'Appearance' and you sould see mytheme in the list of themes.
Enable that and set to default!