If you like trying different beautiful themes on your Gnome desktop, you may would like to clean away those useless themes. In right-clicking on Desktop - > Change Desktop Background -> Theme, there’s a Delete button at bottom, but it is un-click-able.

Actually, it’s just a user permission problem. To solve this, run this command on terminal

gksu gnome-appearance-properties

This will open the previous window with the super user privilege, and you can click Delete button to remove the themes you don’t want to use anymore.