empty cart sidebox background colour

[mediathing]

Hi,
i am trying to change the background colour of the sideboxes, firstly I'm trying the empty cart sidebox.

Having searched through the css, i thought that this would work:

#cartboxempty {
background-color: #333399;
}

But it doesn't work. I think that the above css id may only apply to the text in an empty cart.

Any ideas guys/gals?

Pete