I'm pretty sure you can edit the texture files, but the extent and variety I'm not sure of. You'd probably have to fiddle about with the climate to add "warped by chaos" or "undead" to the usual temperate, desert, woodland etc.

I don't know how useful this is, but I managed to edit the texture files for the buildings in my mod (they're in the blockset folder). The problem is that they are used for a multitude of buildings, meaning that if you do anything drastic like change the colours, it affects a lot of buildings on the battlemap. I think you can edit the church/cathedral/etc textures without it affecting other structures, since they have a unique architecture.

Just thought I'd throw that in.