Want child menu items to be different style from parent

I'm trying to set up my wife's web site:
http://www.domni.com/TypeInsights2
I've successfully made the main menu items appear "tabbed" by rounding the upper corners. But when I hover over a parent menu item, the drop-down child items have the same rounded structure, which looks strange (as in the "Articles" menu item).
How can I make the child items appear with square corners?

Also, when I select the parent item, the menu item color does not change.

Any clues gratefully appreciated!

-Robin