It's real easy once you get the hang of it. The first one will have you tearing your hair, but after that it is easy. I cheat. I take one of the Auctiva themes that I sorta like and then modify it and save it as a new theme.
I just posted 17 auctions last night with my new Christmas theme that I made. I've got others with custom backgrounds, etc. It's nice to be able to have a theme that not everyone else using Auctiva is also using.
Just don't put too much bells and whistles on it or it will be slow loading.