David Kimura PRO said over 2 years ago on Cropping Active Storage Uploads :
  Since we are using CSS within webpack, (import "cropperjs/dist/cropper.css"), you would also need to add the stylesheet_pack_tag in the application layout. Typically, it would probably be something like this 
<%= stylesheet_pack_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %>