Files structure: HOME_DIR: Bootstrap_4_Drupal_9_beta_2020 -color -color.inc^ -preview.css^ -preview.es6.js^ -preview.html^ -preview.js^ -config --install --schema --Bootstrap_4_Drupal_9_beta_2020.schema.yml^ -css --base --elements.css^ --components --block.css^ --book.css^ --breadcrumb.css^ --buttons.css^ --captions.css^ --comments.css^ --contextual.css^ --demo-block.css^ --dropbutton.component.css^ --featured-bottom.css^ --featured-top.css^ --feed-icon.css^ --field.css^ --form.css^ --forum.css^...
To install this theme on your Drupal project: 01. Download folder Bootstrap_4_Drupal_9_beta_2020 2. Upload with FTP on the directory local folder/core/themes 3. Selectl this theme on your Administration menu