Hey man,I just got done doing this to my forums as well.I will warn you that this is a very hard process if you are new to vB (like i am) as for what you plan to mod, it depends on where you want the new design to be.For me i wanted to customize everything from the forum home pages,thread pages all the way down to the user cp pages.It was a hell of alot.The way i figured it out was by dowloading a theme similar to the ones on that site you posted and had a look through there code.It greatly improved my understanding of how to do it.I am not the most technical vB user here so perhaps one of the masters here will help you out.Good luck

and very nice work so far design is looking awsome.