E agora o codigo do javascript:

(...)
// Stylesheet toggle
function setActiveStyleSheet(title) {
  var i, a, main;
  for(i=0; (a = document.getElementsByTagName("link")[i]); i++) {
    if(a.getAttribute("rel").indexOf("style") != -1 && a.getAttribute("title")) {
      a.disabled = true;
      if(a.getAttribute("title") == title) a.disabled = false;
    }
  }
}
function getActiveStyleSheet() {
  var i, a;
  for(i=0; (a = document.getElementsByTagName("link")[i]); i++) {
    if(a.getAttribute("rel").indexOf("style") != -1 && a.getAttribute("title") && !a.disabled) return a.getAttribute("title");
  }
  return null;
}
var curStyleSheet = getActiveStyleSheet();
function toggleActiveStyleSheet() {
  var i, a, main;
  (curStyleSheet == "Ele") ? curStyleSheet = "Ela" : curStyleSheet = "Ele";
  for(i=0; (a = document.getElementsByTagName("link")[i]); i++) {
    if(a.getAttribute("rel").indexOf("style") != -1 && a.getAttribute("title")) {
      a.disabled = true;
      if(a.getAttribute("title") == curStyleSheet) a.disabled = false;
    }
  }
}
function getPreferredStyleSheet() {
  var i, a;
  for(i=0; (a = document.getElementsByTagName("link")[i]); i++) {
    if(a.getAttribute("rel").indexOf("style") != -1
       && a.getAttribute("rel").indexOf("alt") == -1
       && a.getAttribute("title")
       ) return a.getAttribute("title");
  }
  return null;
}
function createCookie(name,value,days) {
  if (days) {
    var date = new Date();
    date.setTime(date.getTime()+(days*24*60*60*1000));
    var expires = "; expires="+date.toGMTString();
  }
  else expires = "";
  document.cookie = name+"="+value+expires+"; path=/";
}
function readCookie(name) {
  var nameEQ = name + "=";
  var ca = document.cookie.split(';');
  for(var i=0;i < ca.length;i++) {
    var c = ca[i];
    while (c.charAt(0)==' ') c = c.substring(1,c.length);
    if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
  }
  return null;
}
window.onload = function() {
		Fat.fade_all(); //fade all
		//fader effect
		fadder = new fx.Opacity('box');
		$('fade').onclick = function() {
			fadder.hide();
			toggleActiveStyleSheet();
			fadder.toggle();
			return false;
		};
		var cookie = readCookie("style");
		var title = cookie ? cookie : getPreferredStyleSheet();
		setActiveStyleSheet(title);
	}
window.onunload = function(e) {
  var title = getActiveStyleSheet();
  createCookie("style", title, 365);
}
var cookie = readCookie("style");
var title = cookie ? cookie : getPreferredStyleSheet();
setActiveStyleSheet(title);

E o codigo html usado para fazer a mudança:

<a href="#" id="fade" accesskey="v" tabindex="7" title="Virar a folha"><u>V</u>irar a folha</a>