That's really all there is to it. The floors are plain white, unless the floor is "above sea level". By this I mean floors at the very base of a level are white (for example, the floor what's supposed to be covered in leaves in the first level), but surfaces of stair steps show up.
Found a better HTP at dengine.net/addons. Haven't tried it with any of the "3D Model" packs or the "Hexen Rain" addon yet.
I think I got the original set at the place I got the HQ music packs for Heretic and Hexen, which would have been a year ago.