How do I change the default theme with Drush-master I've installed a premium theme in a Drupal 8.1.0, and now each time I log into the site It crashes with the error "An unknown error has occurred" (even though the site itself works fine). I thus try to change it with Drush.

I've tried drush vset bartik in drush-master to change the non-admin theme back to bartik in Drupal 8 but then came the error:


  [error]Command variable-set requires Drupal core version 6 or 7 to run.
  [error] The drush command 'vset bartik' could not be executed.
Try this:

drush config-set system.theme default bartik



You might need to clear or rebuild your CSS/JS and theme caches (drush cr).
If the problem still appears, check that it has not been forced in your
settings.php file.  


Source: https://www.drupal.org/node/200774