Grid layout for archive

I am considering messing with some CSS to get a grid layout for my categories/archives at my loadedkiln.com blog. Here's what I have now: http://www.loadedkiln.com/category/s...mic-sculptors/ and I am trying to get a grid instead of a list like this image:



which is from this thread: http://wpmu.org/wordpress-archive-grid/

My question is, based on that guys tutorial, do I add code to the style.css or add the code to the Atahualpa css inserts?

Note: I haven't found a plugin that looks as clean as the above example, which is why I'm even attempting to mess with css.