Thanks to Mootools (my favorite JS Framework), it is possible to change the current CSS stylesheet dynamically really easily :
The Head :
The Body :
The JS:
$('styleSelector').addEvent('change',function(){
var sexyColor = new Asset.css(this.value+'.css',{'id':'colorStyleSheet'});
sexyColor.replaces(window.document.head.getElementById('colorStyleSheet'));
})