nopcommerce css design is beautiful : Is there any resource online ?

Hi,
I was looking at the general design and implementation of nopcommerce classes and of style sheets.

I'm really impressed, I never saw a so beautiful  use of css classes and of style sheets rules.

Is there any resource or link which I can read to understand exactly how planning the development of a web app in a similiar way ?

thanks for any help

Tommaso