How do I 'reset' a theme completely?
If it's a core/contrib theme, just download the original and replace the modified version with it.

If it's a custom theme, just restore from whatever backup you have available (or from Trash or your OS's equivalent feature).

If you're wondering if Drupal somehow keeps track of file system changes and might be able to help you restore deleted files - it doesn't (I've yet to come across a framework that would do this).