Header opacity problems

I have a vision for my header that seems simple in theory but has stumped me so far. I have an image and menu in my header, this is part of a modified twentyten theme. I added a grey background in #header so it is behind both the image and the menu. I would like to take that background and make it transparent. When I add opacity: 0.75; to the style.css file under #header it does turn the background transparent but it also turns my image and menu text transparent too.
How can I keep the image and menu opaque while having the header background color semi-transparent?