Magento2 – How to Uninstall Theme Using Command Line

If after clearing var/cache, var/generation, var/page_cache and pub/static/* folders still you find it there, there is an entry for themes in database table theme with your theme info. Just delete the row containing your theme info and it will be gone from your system.

I was not able to find a direct Magento CLI command so far, I could work around with a Bash and Magerun:

$ THEME_ID="$(n98-magerun2.phar dev:theme:list --format=csv \
  | grep 'Vendor/code' | cut -d, -f1)" \
  ; test -n "${THEME_ID}" \
  && n98-magerun2.phar config:set design/theme/theme_id "${THEME_ID}"

This sequence sets the configuration path for the theme_id to the one of the named theme (by it's code, which is "Vendor/code" in the example).

In case the operation fails, this has a non-zero status.