box <-
fill: colour|gradient|image|{foreground,background}*If a colour object is
specified, the box is filled using a solid colour. A gradient fills it
with a colour transition. An image is repeated to fill the box, starting
at the origin of the window or, if box<-fill_offset
is not @nil,
at that offset from the box. If the initial value @nil
is used the box is transparent. The reserved names foreground and
background use the current foreground and background colours from the
context.
Defaults: @nil
(transparent)
