Deleting Themes

Here's a good post on how to delete themes. (See the post referring to the "how to install anything" page on how to install themes) Basically, to uninstall a theme such as Mist, run the following command:
sudo rm -R /usr/share/themes/Mist

However, some themes are in your home folder in the .themes directory. So change that in the command above.

Another way is to start nautilus under root by running the following command:
sudo nautilus
Then go to the .themes directory in your home folder or /usr/share/themes/ and delete the appropriate theme folder from there.