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.

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?