How to change this one piece of css? Can't find the class

Hi guys, just playing with the menu...and theres one class I can't find with firebug...

When you mouse over a menu item, and get a drop down menu...the menu item has a white shadow around it...I would like to remove this, or change the colour.

If you see http://www.trigeminalneuralgia.co/ you can see what I mean, it doesn't quite look right...

Please can someone tell me where this is coming from? I've tried lots of different parts of the .css with no luck..