If you like trying different beautiful themes on your Ubuntu 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. Run following command in Applcation -> Accessories -> Terminal (or in Alt+F2 pop-up window):
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.