Another problem is that the original concept was that a stylesheet would merely modify and enhance the basic HTML layout tags.
So you could use a page formatted with <H1> and <P> and <PRE> and <TABLE> (and the rest) - and simply swap stylesheets to get a whole new look.
The reality is that to get the most out of CSS you have to realize that pages just aren't made the way God intended, as a simple hierarchical "document." By this I mean something more like Word documents than like Publisher documents.
Since the web looks a lot more like Publisher output today, with pages divided up with complex arrays of banners, navigation strips, sidebars, etc. etc. this simple concept of CSS just didn't cut it.
VERY rapidly things like style classes were added, which means that stylesheets are typically very customized to a page layout and implement ad hoc style classes which must be used within your HTML elements.
You could have 100 stylesheets that could all be successfully applied to one HTML page, but all 101 files would have to conform to the same style class naming conventions and semantics.
So I'm afraid you can't just grab a stylesheet like one might grab a FrontPage Theme and apply it to a given set of HTML pages. Even FrontPage themes use a predefined set of classes and predefined uses for those classes, along with the other items (images, layouts) used to make up the whole "theme."
Sorry, but I don't think you're going to find anything besides ideas (still a good thing) in other people's CSS files. For example you might find a set of font sizes and faces that work together on a page, or a color scheme, or something like that.
Don't give up trying though - there is a lot to be gained by reading other people's work.