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.