[smwhreyebelong]

A better approach might be to define overriding classes and include different stylesheets.

I coul have a .sidebar class styled one way in one stylesheet (say base-stylesheet) and the same .sidebar class styled another way in another stylesheet (skin-stylesheet).

This is a much better way of skinning things than using the other approach where you have to modify the markup and add a new class for every new skin/page. In the overriding case, one has to make sure to load the stylesheets in a specific order.