Download theme file (for example, look at gnome-look.org)
unpack the archive and put the folder with the theme files
on your device. (For example, I downloaded
clock-transparent from gnome-look and put this folder
on my device in /home/user/MyDocs/). Than
- open the settings dialog
- uncheck button "built-in theme"
- push "custom theme" button
- select the folder where your theme files are
(in my example: select the folder clock-transparent)

regards
Nicolai