back ground color
Using Jamroom
menu css should be controlled by the menu.css file (could be core_menu.css), look for the #menu class and change the back-color attribute.
NB if background-color isn't there, it means the menu has inherited the white background, so just add the line in the class.
background-color: #000000;
000000 is black, get the color's hex code you want from here http://www.color-hex.com/
If you're using sitebuilder menu I don't know of the top of my head what the css class is, and my laptop screwed up yesterday and I had to do a reformat so still setting everything up again.