<div class="bgoverlay"></div>

Then in your css add the following:

.bgoverlay{
    position: fixed;
    left: 0;
    right: 0;
    width: 100%;
    height: 100%;
    background: #000;
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=70)";
    filter: alpha(opacity=70);
    -moz-opacity: 0.7;
    -khtml-opacity: 0.7;
    opacity: 0.7;
    z-index: 1;
}
#maincontainer {
position:relative;
z-index:2;
}