var accordion = false;
window.addEvent('domready', function() {
	var stretchers = $$('div.atStart');
	var togglers = $$('h2.atStart');
	accordion = new Accordion(togglers, stretchers, {
		display:0,
		opacity: false,
		onActive: function(togglers, stretchers){
			togglers.addClass('open');
		},
		onBackground: function(togglers, stretchers){
			togglers.removeClass('open');
		}
	}, $('accordion-sidebar'));
});

//Explanation:
//There are three approaches here. In mine you are adding a class to all open togglers. You are able to create a separate css rule for the open bottom toggler by refering to it's ID and the class name, i.e., #accordion-sidebar h2#accordion-blue.open where open is the class you have added.
//In the second approach you are only adding a class to the last toggle when it is open. So you do not have to include your toggler's ID in the css rule, just the class name.
//In the third way you are circumventing the css entirely and changing the background image of the last toggler in the DOM when it is open with setStyle.
//All three will work.