Change background color on Mobile Site nav

Site URL: http://www.dasheprints.com

I can't change the nav background color on my mobile site. It's linked to another part of the code somewhere. I just want the background to be white when the nav comes up on my mobile site instead of gray. Can you help me with code and tell me where to put it?

Add to Design > Custom CSS

/* Mobile menu color */
@media screen and (max-width:880px) {
div#mobileNav {
    background: white;
}
}