Thanks to Mootools (my favorite JS Framework), it is possible to change the current CSS stylesheet dynamically really easily :

The Head :

The Body :

Chocolate
Grey

The JS:

$('styleSelector').addEvent('change',function(){
	var sexyColor = new Asset.css(this.value+'.css',{'id':'colorStyleSheet'});
	sexyColor.replaces(window.document.head.getElementById('colorStyleSheet'));
})