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):

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.

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