how can you get a refund if a theme does not work
Yep, contact support. But if a theme doesn’t work, it would not be accepted for sale on ThemeForest. Are you sure you have installed it correctly? Did you read the documentation? Are you sure you bought a theme (WP, Drupal, ...) and not just an HTML template?
Of course I’m sure. I’m not stupid! also the developer has updated the documentation after my bad experience. Apparently the theme works, but it’s very slow compared to other themes, and this produces errors in the log file