You need to change the color in your Style Manager > find your style in the list > select main CSS from the drop-down. Then, find the class vbmenu_control (it happens to be under the heading of Popup Menu Controls, but you can see the class under CSS Selector: .vbmenu_control) and change it in the background. Put a color in there, or a url to the background gif. Here is an example for one of mine:
Code:
#E4E7F5 url(images/gradients/gradient_panel.gif) repeat-x top left
you can google "css background" to find more css options.