When you try to install a new theme in the Appearance preferences window, you may receive this error:
This means that the theme you are trying to install is already installed.
If you cannot see the theme anywhere in the Appearances window, it may have been deleted. However, some files could have left behind which prevent it from being installed again.
Press
Type nautilus .icons and click
Delete the folder which corresponds to the theme you are trying to install.
Try to install the theme again.
Cursor themes are stored in the