How can I add an external (CDN) CSS style sheet to a theme? I'm trying to add Font Awesome to a Bootstrap subtheme I have made.

Here is the code I am using in subtheme.libraries.yml, which is based on the documentation for hosting external JS libraries in Drupal 8:

font-awesome:
  remote: https://fortawesome.github.io/Font-Awesome/
  version: 4.5.0
  license:
    name: MIT
    url: https://fortawesome.github.io/Font-Awesome/license/
    gpl-compatible: true
  css:
    https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css: { type: external, minified: true }


Unfortunately this results in a WSOD.  The error log gives these two errors:


  Warning: constant(): Couldn't find constant
  CSS_HTTPS://MAXCDN.BOOTSTRAPCDN.COM/FONT-AWESOME/4.5.0/CSS/FONT-AWESOME.MIN.CSS
  in Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension() (line
  143 of
  /srv/bindings/code/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php).


And:


  Warning: Illegal string offset 'weight' in
  Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension() (line 143
  of
  /srv/bindings/928c941920f8443d9e50a3d908967fee/code/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php).
My understanding was that for a sub theme you defined the CSS as such: 

global-styling:
  css:
    theme:
      css/style.css: {}


...as discussed here: https://www.drupal.org/node/2165673