back-end checkboxes not displaying after switch to Shoreditch
I've discovered that the Shoreditch README states that "At the moment the theme is being developed to work only in Drupal." The CiviCRM extension page for Shoreditch, however, states, "CMS Compatibility: 
CMS Independent (CiviCRM-native)". Shoreditch, however, is still required for Mosaico, which does work correctly with Joomla.

So, apparently, my expectations that Shoreditch will work correctly as the Civi theme with my Joomla installation are incorrect, and I'll move back to my custom css.

It would be helpful if the extension page would clarify what Shoreditch is expected to do and not do for non-drupal users.