Set the background-color for #navSuppWrapper {} instead of dd {}, and adjust the height and margins as desired to get the bottom edge of the bg color where you want it.

You can also set the width of dd{} to 24.91% to make it fill the space exactly (you might need to make it 24.9% to fit in all browsers).

You weren't online when I started to answer, Clyde, so I thought I'd give a quick reply... didn't look at the actual stylesheet, though - Firebug can be misleading:)