Mike,
The NAV your using isn't rendered as a list which is why FireFox and Safari are ignoring the styles you've setup for them.
IE isn't the best browser to be testing in, and many website developers (including myself) will advise you do your build and test in Firefox first, and then check it in other browsers, as it is one of the most consistent browsers with regards to CSS and rendering.
I also think the way you're defining your selectors is incorrect for the way the menu is built .SchoolMenu.root is looking for an element that has both styles applied to it, whereas I think you mean .SchoolMenu .root (with a space between the selectors) because the menu is wrapped in a span with the class of SchoolMenu containing spans with classes of root (this is the same for the other classes you're targeting: .mi, .sel etc).
I think the best thing to do is test this in Firefox, using the Firebug plugin (https://addons.mozilla.org/en-US/fire...) this will let you view the rendered code (bear in mind that this is not the same as the source code, because it uses javascript to inject the various classes). From this you'll be able to setup the CSS correctly.
If you can, get the menu to render as a list as this is easier to work with because the root items are one list and sub menu are another. Timo has an example of this available here: http://dnnskinextensions.codeplex.com...
Please let me know if this helps, or if there's anything else.
Regards,
Rick.