datetoday = new Date();
timenow=datetoday.getTime();
datetoday.setTime(timenow);
thehour = datetoday.getHours();

if (thehour >= 18)  
	display = "../css/night.css"; 
else if (thehour >= 16)  
	display = "../css/day.css"; 
else if (thehour >= 6)  
	display = "../css/day.css"; 
else if (thehour >= 0)  
	display = "../css/night.css";
else 
	display = "../css/day.css";

var css = '<';
css+='link rel="stylesheet" href=' + display + ' \/';
css+='>';

document.write(css);