window.onload = setScreenClass; 
window.onresize = setScreenClass;

//  Following transition classes will be declared:

//	classname		  screenwidth
//	---------------------------------------
//	default	under800	  0px 	> 800px
//	default over800		 	> 800px

function setScreenClass(){
	var fmt = document.documentElement.clientWidth;
	var cls = (fmt>=800)?'default over800':'default under800';
	document.body.className=cls;
};