From http://sldn.softlayer.com/blog/cwolff/jQuery-and-CSS-What-time-it-you

different-css-day-time.css

.evening {background: #999;}
/* ... */

different-css-day-time.html

<script type="text/javascript">
$(document).ready(function () {
    datetoday = new Date(); // create new Date()
    timenow = datetoday.getTime(); // grabbing the time it is now
    datetoday.setTime(timenow); // setting the time now to datetoday variable
    hournow = datetoday.getHours();  //the hour it is

    if (hournow >= 18)  // if it is after 6pm
        $('body').addClass('evening');
    else if (hournow >= 12) // if it is after 12pm
        $('body').addClass('afternoon');
    else if (hournow >= 6)  // if it is after 6am
        $('body').addClass('morning');
    else if (hournow >= 0)  // if it is after midnight
        $('body').addClass('midnight');
});
</script>