var curTime = new Date ( );

var curHours = curTime.getHours ( );
var curMinutes = curTime.getMinutes ( );
var curSeconds = curTime.getSeconds ( );
  
if (curHours >0 && curHours<13){
	document.write('<link rel="stylesheet" type="text/css" href="css/maincss.css" />');
} else if (curHours >12 && curHours<18){
	document.write('<link rel="stylesheet" type="text/css" href="css/maincss.css" />');
} else if (curHours >17 && curHours<24){
	document.write('<link rel="stylesheet" type="text/css" href="css/maincss_night.css" />');
} else if (curHours==0){
	document.write('<link rel="stylesheet" type="text/css" href="css/maincss_night.css" />');
}