I've installed a premium theme in a Drupal 8.1.0. Each time I log in, the site crashes with the the following message.
An unknown error has occurred.
I then tried to change the default theme using
drush vset bartik to change the theme for non administrative pages back to the Bartik theme.
I got this error.
Command variable-set requires Drupal core version 6 or 7 to run.
The drush command 'vset bartik' could not be executed.
How should I change the default theme with Drush?