How can I change the Menu background color?

I've changed the background of the site, and want to have the menu background change to white (With the text changing to black)

I looked at the style.ccs and thought I found where it was.
.sf-menu { position: relative; margin: 0; padding: 0; list-style: none;clear:both; float: left; margin-top: 12px; padding-left: 9px; margin-bottom: 1em; background: url('images/menu_bg.png') no-repeat; height: 58px; width: 957px; z-index: 400; }

I edited the "menu_bg.png" and uploaded it back to the server, but nothing has changed.

Am I looking in the right place, or do I have to modify the code ?

Thanks for the help!