window.onload = function () {
    var currentDate = new Date();
    var currentHour = currentDate.getHours();

    // background from 7pm to 5am
    if ((currentHour >= 0 && currentHour <= 5) || (currentHour >= 19 && currentHour <= 23)) {
        document.getElementsByTagName("body")[0].className = "bg_sunset";
    }

    // background from 6am to 8am
    else if (currentHour == 6 || currentHour == 7 || currentHour == 8) {
        document.getElementsByTagName("body")[0].className = "bg_sunrise";
    }

    // background from 9am to 6pm
    else if (currentHour >= 9 && currentHour <= 18) {
        document.getElementsByTagName("body")[0].className = "bg_day";
    }
}