Unlike some other desktop environments, Xfce does not currently have a separate application to help you automatically install new desktop themes. Instead, to use a new desktop theme, you need to store new themes in a special folder inside of your home folder. The following instructions will show you how to install and use a new desktop theme in Xubuntu.
Download a theme from the internet, and save it to your computer. Xfce-look.org is a popular site that contains many themes that are compatible with Xubuntu. Additionally, many of the Gtk-2 themes from gnome-look.org will also work with Xubuntu.
Create the .themes folder. Inside of your home folder, create a new folder and name it, ".themes". Thus, the full path to this folder would be → →
Extract the theme archive into your .themes folder. Using Thunar, navigate to the folder where you downloaded your theme archive and double-click on the archive that contains your new theme. This will launch the file-roller application. Use the file-roller application to extract the theme into a your newly-created .themes folder.
Select the new theme. After completing these steps, your newly installed theme should be available to select in → → .