No info.yml file in themes for Drupal8

I just bought a theme for a new Drupal 8 installation.
After downloading it I tried to upload it straight on my Drupal site.
It gave me an error saying that there is no info.yml file in the Theme.

I tried uploading the theme via FTP into the folder “Theme”. Now I could select it in the themes under “appearance”, but when I select the theme as default, the setting page and front page is a white screen.

What can I do to get my 50$ theme to work? It’s really shouldn’t be this difficult.