The background colour can be set either using a GUI dialog
(Edit$ -> Background Colour) or the function
(set-background-colour 0.00 0.00 0.00), where the arguments
are 3 numbers between 0.0 and 1.0, which respectively represent the
red, green and blue components of the background colour. The default
is (0.0, 0.0, 0.0) (black).