I've installed Xmenu, and tweaked Style12 to fit the skin for this project - otherwise I haven't made that many other changes to the code. However, when I started working on the sub-menu styling, I noticed that the submenu seems to have an extra ~10px top padding, forcing it to overlap it's border.

Also, when the pages from the sub-menu are selected, the "tab" seems to inherit the width of the page name, but still positioned over the root item.

The skin is still in heavy development, but you should be able to view it in action at: http://tctcbookstore.com.netdocsdev.com/
Some other quick details:
DNN 5.4.2
Doctype is XHTML 1.0 Transitional