Let`s learn how to switch Drupal 8 theme. Go to "Appearance" page and click "Install new theme". Open in the new tab bootstrap theme page https://www.drupal.org/project/bootstrap and copy link to tar.gz opposite 8.x
and paste the link to the "Install from a URL" field and click "Install"
After that click "install newly added theme"
Scroll to bottom of the page and click "Install and set as default" near bootstrap theme. Theme should be installed now, you can check by opening front page.