This will change the background on the navigation menu to the grey you specified.
One other thing I noticed is that your background image is 8400px x 8400px at 300 DPI and is nearly 18MB file size. It takes a very long time for your site to load for me on my 3Mbps internet connection.
I would suggest downsizing the image in your image editing program to around 2500px square (very few people have monitors over 2000px in width) and setting it to 72 DPI, which is the web standard and then saving it as a medium to low quality JPG. Shoot for a file size of 250k or less and let that be your guide for the JPG compression/quality setting.