html {
    background: url("img/bg/bg_dark_1200x900.png") no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
    filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src = 'img/bg/bg_dark_1200x900.png', sizingMethod = 'scale');
    -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='img/bg/bg_dark_1200x900.png', sizingMethod='scale')";
}

body {
    background: url("img/bg/bg_dark_1200x900.png") repeat-x center center fixed;
}