In Xubuntu I believe it is a matter of clicking on the mouse, followed by Settings | Appearances. I can't check right at the moment because Clonezilla is working its magic on my Xubuntu box, but that's the ticket if memory serves. Click on settings and then click on either Appearances or Desktop, one of the two.
My desktop runs 64-bit Linux Mint Nadia KDE, my htpc runs 64-bit Linux Mint Nadia Xfce, my answering machine runs 32-bit windows xp, and my laptop runs 64-bit Linux Mint Nadia KDE. Each seems suited to its purpose.