Customize the Impact Theme

It's me again. Like i said in my previous topic, i like the impact theme and i have never heard of css before last week. There are different things i would like to customize, because the website is for a swiss research institute (can't say which one in the moment) At the current status there are now 4 themes to evaluate (impact is one of these) . Could someone give me something like a manual on the impact theme? Something like: this part of the style.css refers to this part of the website.