bootstrap.min.css
und bootstrap.min.js
heunterladen, z.B. aus :
https://cdn.jsdelivr.net/npm/bootstrap@3.4.1/dist/css/bootstrap.min.css
https://cdn.jsdelivr.net/npm/bootstrap@3.4.1/dist/css/bootstrap-theme.min.css
https://cdn.jsdelivr.net/npm/bootstrap@3.4.1/dist/js/bootstrap.min.js
Drupal Custom Theme Library anpassen: