There's comp.infosystems.
www.authoring.stylesheets
CSS source is partly in element attributes (style="...") and in <style>
elements, which you see when you view HTML source, partly in external
stylesheets, which are referred to by <link> elements, which you also see in
HTML source, so you would need to find out the absolute URLs of external
stylesheets and access them with your browser.
More easily, use Firefox and install the Web Developer Toolkit. It has nice
CSS tools, for viewing all stylesheets directly, for viewing styles that
apply to a particular element, and even for editing CSS code on the fly. I
can't imagine how I lived years without it.