How can the nav background color change?

Appearance > Theme Options > CSS Styles

Add this code:

.menu {
    background-color: #009900;
    background-image: none;
}

Just edit the #009900 color I added

I tried that as well as

.main-nav

what finally worked for me was adding in a ul

.main-nav ul { background:#fcfcfc; }

Oh, you wanted the sub-menu color changed?

no the top level background

And you added the ‘background-image: none’ also?