Drupal not loading theme CSS with HTTPS
For a Drupal 7 site using Cloudflare CDN and a Bootstrap theme, this worked for me: Set the base URL in your settings.php file, without including http:// or https://.

$base_url = '//my.domain.name';


For another Drupal 7 site with a different theme, just removing $base_url worked.
In both cases, I needed the site to work with secure URLs and without.