.site-gray {
    /* IE6-9 */
    filter: gray;
    filter: grayscale(100%);
    filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale='1');
    /* Chrome 19+ & Safari 6+ */
    -webkit-filter: grayscale(100%);
    -moz-filter: grayscale(100%);
    -ms-filter: grayscale(100%);
    -o-filter: grayscale(100%);
}

.site-gray body {
    filter: gray;
}

.site-gray img {
    filter: gray;
}