function getCssFromTime()
{
    var today = new Date();
    var hour = today.getHours();

    if (hour > 5 && hour < 18){//早上五時後至下午六時前
        show = "css/day_main.css";
    }else{
        show = "css/night_main.css";
    }

    var css = "";  
    css='<link href="' + show + '" rel="stylesheet" type="text/css" />';  
    document.write(css);

}