All themes come with a /css/ directory.
Inside you'll find either a style.css or custom.css you can freely edit as you fancy
In these files, CSS declaration are grouped together , e.g. *Price tag*, *Item*, *Footer* , etc...
Use a free HTML editor or the powerful CSS editor called
TopStyle to get a kind of "WYSIWYG" preview before you upload to your site host
Hope it helps
With Regards
Eman