How do I 'reset' a theme completely? I think I have deleted some .css theme files in my local directory by accident and when I delete All the theme's files to re-install and get the original files back, they still aren't there. Is there a way to reset the whole theme folder?
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).