Configure absolute paths
Start by going to Administer > System Settings > Resource URLs.
Make the following settings:
- Force Secure URLs (SSL).
- CiviCRM Resource URL: https://yoursite.com/administrator/components/com_civicrm/civicrm/
- Image Upload URL: https://yoursite.com/media/civicrm/persist/contribute/
- Custom CSS URL: https://yoursite.com/templates/protostar/css/crm.css
- Extension Resource URL: https://yoursite.com/media/civicrm/ext/
Note the Custom CSS URL location. I've put it in the protostar template CSS folder, but you could put it anywhere that accessible. Also it's important to note that most of the URLs require a trailing forward-slash, but naturally this isn't applicable for the crm.css file.
Thanks for visiting,