Issue #2575581 by bryanbraun: Update APIs to use several new Drupal 8 paradigms.

Including:
  - Move default theme settings to config/install/writer.settings.yml
    (see also https://www.drupal.org/node/2382645)
  - Change the check_plain reference to Html::escape (see also
    https://www.drupal.org/node/2457593)
  - Change code-snippet CSS files into libraries (see also
    https://www.drupal.org/node/2379475)