drush gesso "New Theme Name" ignores custom folder

Hi,

Thanks for the sub theme!

My setup
I manually created a themes/custom folder and a themes/contrib folder. My website runs on a composer based Drupal install (composer create-project drupal-composer/drupal-project:8.x-dev projectname --stability dev --no-interaction) locally with DrupalVM. The VM I'm running holds multiple sites. The structure is of my VM is:
docroot/websites/website1/web/drupal_code
docroot/websites/website2/web/drupal_code

My issue
When I run drush @site-alias gesso "New Theme Name" the theme gets created in the themes folder instead of the themes/custom folder.

Maybe this issue is caused by the setup of my VM. Or the fact that I didn't use composer to install the theme but Drush. I cannot test this because I'm in Indonesia and on a very slow Internet connection. But in gesso.drush.inc I don't see a variable for location. I don't know enough about Drush code to determine if this is the cause.

Jasper