if you have a picture or a color that you would like the background to be you could say:

  <b>_SCREEN.PICTURE</b> = mypic   && this is for a picutre
    <b><i>OR</i></b>
  <b>_SCREEN.BACKCOLOR</b> = RGB(12,34,255) && this would be any color of the RGB table or some custom color   

if you had a picture you would need to check the size of the screen resolution and make your picture the size of the screen.

then depending on what size the resolution, you could give the appropriate backgroud picture for that resolution