Install drupal theme with composer

• Go to the themes homepage to copy the the official composer command to run

cd yourprojectdir

Install with composer

composer require drupal/mayo

Enable the theme in the backend

• go to /admin/appearance
• enable and set as default in the admin

Clear cache with drush

You will probably see an error unless you clear the cache with drush

drush cr

Enable theme with drush (optional)

You cannot download and install themes with drush. However, you can set the theme as default

drush en mayo