Try ‘colour map-bg transparent’ in your layout.  Around pg 54 of the Therion Book.

If it is working it should give you a pdf of the cave with a transparent background.  I have not tested this recently.  When it was first brought in, around version 5.3.5, it had to be placed directly in the export statement in your thconfig, something like;

export map -layout-colour map-bg transparent

Bruce