I have to say that Joomla does this very nicely. Now, I hate to be one to call out another CMS as an example ("Well, if you were just more like..." is not my intention). They have their drawbacks, and learning from their strengths is a good thing.

Every piece of Dolphin code, every module, every add-on should have two parts. The part that gets installed in the directory where it needs to be to function as a part of Dolphin, and the part that gets dropped into /templates/[active-template-directory]/module-name.css.

Even see more Joomla does not force these standards, causing a lot of cleaning up on CSS code whenever a new module is installed, either because of css files being spread all over or because of inline (throws up a bit in the back of my throat) styling added into the code itself, rather than a standalone CSS sheet. *cries*

To make it two-part like this as a standard would allow some pretty amazing things, design-wise, and be amazing for Dolphin, IMHO.