How do I edit css for the shopping cart page?

Hello, I am brand new to MarketPlace (just purchased today). I am wondering how to style my shopping cart to look like the sample below?
http://digitaldownloads.wp-ecommerce.org/store/shopping-cart/

Currently my header and shopping cart content is all squished together: http://off.olympiawebsitedesigns.com/store/shopping-cart/ If it helps, I am using Thesis 2.0. I am very familiar with CSS syntax, just not sure which ids and classes to style and where to find them. I mean, I know I can go to the marketplace style.css file, but any update would overwrite it, correct?

Thanks!