function fontsizeup() {
  active = getActiveStyleSheet();
  switch (active) {
    case 'A' : 
      setActiveStyleSheet('A+');
      break;
    case 'A+' : 
      setActiveStyleSheet('A++');
      break;
    case 'A++' :
	 	setActiveStyleSheet('A+++');
      break;
    case 'A+++' :
	 	setActiveStyleSheet('A++++');
      break;
    default :
      setActiveStyleSheet('A++++');
      break;
  }
}

function fontsizedown() {
  active = getActiveStyleSheet();
  switch (active) {
	 case 'A++++' : 
      setActiveStyleSheet('A+++');
      break;
	 case 'A+++' : 
      setActiveStyleSheet('A++');
      break;
    case 'A++' : 
      setActiveStyleSheet('A+');
      break;
    case 'A+' : 
      setActiveStyleSheet('A');
      break;
    default :
      setActiveStyleSheet('A');
      break;
  }
}

<!-- CHANGE ACTIVE STYLE SHEET -->
function setActiveStyleSheet(title) {
  var i, a, main;
  var image = new Array(); 
  image[0] = "images/font1.jpg";
  image[1] = "images/font2.jpg";
  image[2] = "images/font3.jpg";
  image[3] = "images/font4.jpg";
  image[4] = "images/font5.jpg";
  
  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;
		}
    }
  }
  var active = getActiveStyleSheet();
  var x;
  switch (active) {
    case 'A' : 
      x = document.getElementById("fontguide");
		x.src = image[0];
      break;
    case 'A+' : 
      x = document.getElementById("fontguide");
		x.src = image[1];
      break;
    case 'A++' :
	 	x = document.getElementById("fontguide");
		x.src = image[2];
      break;
    case 'A+++' :
	 	x = document.getElementById("fontguide");
		x.src = image[3];
      break;
	case 'A++++' :
	 	x = document.getElementById("fontguide");
		x.src = image[4];
      break;
    default :
      break;
  }
}

<!-- GET ACTIVE STYLE SHEET -->
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 'A';
}

<!-- SET DEFAULT STYLE SHEET -->
function getPreferredStyleSheet() {
  return ('A');
}

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(e) {
  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();
if (title == 'null') {
  title = getPreferredStyleSheet();
}