There is a lively discussion on the DotNetNuke Core Team forums about DNN stylesheets. If there is one area of DNN that I don't care much for, it would be in how DNN handles stylesheets. It's a bit messy and leaves even the most CSS-savvy designer scratching their head about how/where/why a particular style was inherited by a particular element.

Personally, I don't think there is a need for host, portal, skin, container and module stylesheets. There should be one stylesheet used on the page and that is the skin stylesheet. All classes required by a page where the skin is applied should be defined in this stylesheet. While this may require a bit more work, it is definitive and portable.