You can start by looking at the existing class designations for the OpenACS standard CSS sheets in CVS HEAD. You'll find things like "portal", "portal-head", "portal-body" which are shared by Lars's new default index page for acs-subsite and my "top-secret" (meaning mostly I was busy on other things in Guatemala) work on portals (the standard portlet style uses the master template css classes).
There's a bunch of other classes, like for stuff spewed by the templated list utility Lars wrote not too long ago.
So ... Jade's right - don't get ahead of yourself! We need to build on the incremental progress already made in moving towards the definition of standard css classes in a logical, well thought-out way.
When CSS stuff is properly defined and we have a reasonable default design for OpenACS and .LRN we can then start looking to supplying a library, if you will, of other designs folks can plug in just by switching CSS files.