var myrules = {
	'.form_button' : function(el){
		el.onmouseover = function(){
			shiftBackground(this);
		};
		el.onmouseout = function(){
			shiftBackgroundBack(this);
		}
	}
};

Behaviour.register(myrules);

function shiftBackground(elem) {
	elem.style.backgroundPosition="0 -26px";
};

function shiftBackgroundBack(elem) {
	elem.style.backgroundPosition="0 0";
};