[Therion] thfill with page background colour?

Hi metapost wizards, I try to fill a label with the page background. thfill ((bbox lab)) scaled (sc * basescale * 1.5) withcolor <insertColorHere>; For <insertColorHere> i tried "label_fill_color", which however may be a different background. I also tried "background", but that yields the cave passage background colour. In which variable do i access the page's background color (the one set wit the layout option "map-bg")? Thanks! Beni