There is no setting for the background color.
However a big white box as a background layer will give the same effect.
Here's a geometry for a world covering box in WGS84, place in load script, use bg_box as dim/geometry
load * inline "
world|'[[ [-180, -80], [-180, 80], [ 180, 80], [ 180, -80], [-180, -80] ]]'
(ansi, txt, delimiter is '|', embedded labels)
To render: change draw order to Geodata layer, switch off the info bubble, don't include in AutoZoom, make the layer non-selectable and set color mode to single color: white.
white_bg.qvw 157.0 K