There seems to be an option to control this, but it doesn't work for me.
In Fabrication Outputs, go to Final Artwork Prints, click Configure, right click on a layer, and choose Preferences. Here you can control the colors - there is a selection for background color, which by default is white.
I set the track to white and background to black, but in the print preview I only got a white page - the tracks changed but the background did not.
What's the purpose of this background color if it can't be changed?