In my JavaScript Pull-Quotes plugin I have a folder of CSS files within the plugin directory. The admin page allows the user to select a style from a drop-down list.

I've also integrated some error checking -- for example, if that folder goes missing it falls back to a copy of the "default" stylesheet kept in a different part of the plugin file.

Finally, if there is a file names "pullquote.css" in the theme directory, that overrides any other setting. Thus: a few options for the point-&-click set, and the ability to completely customize (via theme directory) for the power users. Down the road I may also simply allow the user to specify a styles directory so they can keep the folder that populates the drop-down in a completely different location.