You can't change the background for individual devices. There are 2 backgrounds for devices and 2 for activities. One displays on the main pages, the other displays on the sub-pages. You can locate those files in
\Program Files\Universal Remote Control, Inc\MX-810 ProWizard\swffile\Graphics\
Then the name of the theme and the folder \Elements.
There is a work-around, however. You can create custom icons to fill the whole screen. See the Custom Icon Tutorial:
[Link: remotecentral.com].