Root Background \n

This refers to the background that fills the screen behind all of the windows
.  It's known as the 'root window'.

You can enter a command in the entry
and click on 'Apply'.

You can choose a color with the color selector and click on
the 'root' button on the color selector panel.

You can choose an image with the image selector. The 'root' menu on
the image selector
has several numbered entries. These display the image on the root window
in different ways (using xv or imagemagick , if you have them).