Since the cart is encapsulated in DNN, that means that it can be influenced by a multiple of stylesheets, or inline styles.
The easiest way to see EXACTLY what is influencing a particular element; whether it's a button, a font, an image, whatever, is to use FireFox with a plugin called Firebug.
All you have to do is load the web page into firefox, right-click on what you wish to modify, and then click "Inspect Element". It will show you the HTML, the style sheets, and everything. You can even do "test changes" on the page before going in and modifying the ascx or css files to see how your changes will look.
Needless to say, this works with anything in DNN. It allowed me to style our entire site in a matter of a few days, because there was no guesswork about what stylesheet was overriding another one.