Structure Theme – Inconsistent w/ Firefox and IE

http://christmasjones.com/christmas/

I am having an issue with the Structure Theme. I want to customize the colors to suit my site, so I changed the main body background (not page background) color in the stylesheet:

#body-container {
overflow: hidden;
width: 942px;
margin: 0 auto;
padding: 0 12px;
background: #FFFFFF;

I also changed the main navigation background color:

#nav-container {
overflow: hidden;
display: block;
padding: 0;
background: #6FAC8E;

The only coding I changed was the actual colors- nothing else. The problem is, once these 12 numbers/letters were changed, the background of the navigation menu does not show in Internet Explorer. It’s fine in Firefox.

Any suggestions?