What you are talking about sounds pretty good.

In my way of thinking, the ID specific styling is a great idea, but having the menu load images that have names specific to the menu item would allow a more dynamic implementation of the styling per site.

For example, a rather "static" skin, using a menu which loads images from a designated image folder on each site (looking first in the same folder as the css, then in the designated folder would allow each site owner in the portal instance to "sort of" modify the skin by adding "page specific" images.  Otherwise the default images would load from the css.

I also think that having the ability to "include" or "exclude" pages from a specific instance is critical for flexibility.