If you realize the site is designed with PHP-NUKE which is like postnuke. We use the same thing on our site _lildawg_site_. We use postnuke with a pnPHPBB module added. We then also have x-cart on our site just as this example site does. What heppens in this case with say like postnuke (what we use) is they are both smarty based so if I make a design on one I can make it on the other system. Then a few years ago I became friends with the creator of Xanthia the theme engine now used for post-nuke and was able to get some really good beginner themes which you can then turn into anything.
So if you notice when you first go to the site it is the phpnuke site then when you click on a product it actually then takes you to the xcart site. I also for a customer created a static page with a iframe in it that inturn sticks PHPbb into it. You can see an example by visiting
http://www.smilecamusa.com/store/customer/pages.php?pageid=34. To make it so that when a customer creates an account on x-cart it also creates one in the PHPbb forum I used the phpBB Integration Plus. This allows you to have a PHPbb forum inside your x-cart system without customers leaving your x-cart store.