When I add CSS/JS library, Drupal 8 / Theme not displaying correctly.
If you are activated 'Aggregate CSS files' on admin/config/development/performance, Disable it and refresh the page.
If the page display correctly, Probleme is in CSS Aggregation.
1. Check Permission to write on drupal_site_folder/files/css (exemple drupal/sites/default/files/css)
2. For the moment, you can leave this function disabled (decrease the performance).
File not writable. ("The file could not be created")
1. Check tempory folder on admin-> config -> file system ( Under Media )
2. Check Permission to write on drupal_site_folder/files/css
After all, do not forget to clear all cache (drush cr OR admin/config/development/performance)