window.onload = setScreenClass;

function setScreenClass()
{
    var cls;
    var ranNum= Math.floor(Math.random()*3);
    if(ranNum==0)
    {
    	cls = 'blue';
    }
    else if(ranNum==1)
    {
	    cls = 'yellow';
    }
    else
    {
	     cls = 'red';
    }
	document.body.id=cls;
};