Another option with pure CSS would be something like this:
nav {
margin: 0px auto;
width: 100%;
height: 50px;
background-color: white;
float: left;
padding: 10px;
border: 2px solid red;
position: relative;
z-index: 10;
}
nav:after {
background-color: grey;
content: '';
display: block;
position: absolute;
top: 10px;
left: 10px;
right: 10px;
bottom: 10px;
z-index: -1;
}
Demo here