Navigation: Making one button go to different graphic when on button’s page

I’m trying to figure a way using WP where I can have a CSS based nav menu where depending on the page you’re on, the image for the button representing that page changes to represent the fact the page is selected. In other ways, let’s say you’re on the About Us page. Normally the graphic would be grey on white but on this page, it needs to be white on orange like a tab. How could I make this work in a CMS?