Well, the best place I know of to get GNOME themes is right here
, and for Linux Mint themes right here
Now to install them (after you downloaded the one(s) you want), go to your Control Panel, then click on Appearance. If you downloaded a *.tar.gz file you can just click and drag it into the Theme window that popped up when you clicked on 'Appearance'.
If it is a .deb file you can just double-click on the file and it should install.
To delete these, navigate to ~/.themes, and delete away.
Hope that helps!