Where's the main css for Spirit/Upfront kept?

From the Wordpress dashboard, I went to Editor and scrolled down to StyleSheet (style.css). But when it opens, there's hardly anything in the document. Where is the main css kept?

More specifically, with the Spirit theme, the text entry boxes don't have borders. I have installed MarketPress (and it works pretty good so far), but the drop down menus and quantity boxes on the product pages are pretty much invisible (making it difficult for the customer). I want these to have the traditional border across the whole site, so I was going to edit the master css file, but I can't find it. What's the best way to go about doing this?