First go to Design -> Product listing design. There are three tabs there:
HTML - Deals with the basic structure of the page. On the top of that tab you can select the number of products in a row and add special fields like the price or add to cart button. The bottom panel has the HTML code that will set the layout of a single product cell.
CSS - This is a css file that will be used on all the product detail pages. You should use it only to style the product listing or if you want some other part of the website too look different on those pages.
Images - Is used to upload images to the server.
