function setHeight() {
	if (document.getElementById) {
		var lHeight = document.getElementById('back');
		var winHeight = window.innerHeight;
		if ( lHeight.offsetHeight < winHeight ){
			lHeight.style.height = winHeight + 40 + 'px';
		}
	}
}
window.onresize = function() {
	setHeight();
}
function change(id, newClass) {
identity = document.getElementById(id);
identity.className = newClass;
}