Another way to set things back the way they were, at least for Ambiance and Radiance, is to run:
sudo apt-get install --reinstall light-themes
This will simply restore the Ambiance and Radiance themes to the version that is in the repos.
Also, for the future, it may be better to copy over whichever theme you plan to modify to the
~/.themes hidden subfolder in your home folder. Then, do whatever you want there. (If
~/.themes doesn't exist, just create it.) The advantage is that you have the original as a backup and don't need
sudo to edit any files in
Edit: Thanks to Tachyons for correcting the code to include