Check/Uncheck all boxes using javascript

I have the following page www.crownvalleywinery.com/kiosk/default.html and I would like to add a button or checkbox to check/uncheck all. Note we are already using some javascript for custom checkboxes so it needs to integrate with that. Any help is appreciated.

Here is the current javascript...

/*

CUSTOM FORM ELEMENTS

Created by Ryan Fait
www.ryanfait.com

The only thing you need to change in this file is the following
variables: checkboxHeight, radioHeight and selectWidth.

Replace the first two numbers with the height of the checkbox and
radio button. The actual height of both the checkbox and radio
images should be 4 times the height of these two variables. The
selectWidth value should be the width of your select list image.

You may need to adjust your images a bit if there is a slight
vertical movement during the different stages of the button
activation.

Visit http://ryanfait.com/ for more information.

*/

var checkboxHeight = "47";
var radioHeight = "25";
var selectWidth = "190";

/* No need to change anything after this */

document.write('<style type="text/css">input.styled { display: none; } select.styled { position: relative; width: ' + selectWidth + 'px; opacity: 0; filter: alpha(opacity=0); z-index: 5; }</style>');

var Custom = {
	init: function() {
		var inputs = document.getElementsByTagName("input"), span = Array(), textnode, option, active;
		for(a = 0; a < inputs.length; a++) {
			if((inputs[a].type == "checkbox" || inputs[a].type == "radio") && inputs[a].className == "styled") {
				span[a] = document.createElement("span");
				span[a].className = inputs[a].type;

				if(inputs[a].checked == true) {
					if(inputs[a].type == "checkbox") {
						position = "0 -" + (checkboxHeight*2) + "px";
						span[a].style.backgroundPosition = position;
					} else {
						position = "0 -" + (radioHeight*2) + "px";
						span[a].style.backgroundPosition = position;
					}
				}
				inputs[a].parentNode.insertBefore(span[a], inputs[a]);
				inputs[a].onchange = Custom.clear;
				span[a].onmousedown = Custom.pushed;
				span[a].onmouseup = Custom.check;
				document.onmouseup = Custom.clear;
			}
		}
		inputs = document.getElementsByTagName("select");
		for(a = 0; a < inputs.length; a++) {
			if(inputs[a].className == "styled") {
				option = inputs[a].getElementsByTagName("option");
				active = option[0].childNodes[0].nodeValue;
				textnode = document.createTextNode(active);
				for(b = 0; b < option.length; b++) {
					if(option[b].selected == true) {
						textnode = document.createTextNode(option[b].childNodes[0].nodeValue);
					}
				}
				span[a] = document.createElement("span");
				span[a].className = "select";
				span[a].id = "select" + inputs[a].name;
				span[a].appendChild(textnode);
				inputs[a].parentNode.insertBefore(span[a], inputs[a]);
				inputs[a].onchange = Custom.choose;
			}
		}
	},
	pushed: function() {
		element = this.nextSibling;
		if(element.checked == true && element.type == "checkbox") {
			this.style.backgroundPosition = "0 -" + checkboxHeight*3 + "px";
		} else if(element.checked == true && element.type == "radio") {
			this.style.backgroundPosition = "0 -" + radioHeight*3 + "px";
		} else if(element.checked != true && element.type == "checkbox") {
			this.style.backgroundPosition = "0 -" + checkboxHeight + "px";
		} else {
			this.style.backgroundPosition = "0 -" + radioHeight + "px";
		}
	},
	check: function() {
	
		element = this.nextSibling;
		if(element.checked == true && element.type == "checkbox") {
			this.style.backgroundPosition = "0 0";
			element.checked = false;
		} else {
			if(element.type == "checkbox") {
				this.style.backgroundPosition = "0 -" + checkboxHeight*2 + "px";
			} else {
				this.style.backgroundPosition = "0 -" + radioHeight*2 + "px";
				group = this.nextSibling.name;
				inputs = document.getElementsByTagName("input");
				for(a = 0; a < inputs.length; a++) {
					if(inputs[a].name == group && inputs[a] != this.nextSibling) {
						inputs[a].previousSibling.style.backgroundPosition = "0 0";
					}
				}
			}
			element.checked = true;
		}
	},
	clear: function() {
		inputs = document.getElementsByTagName("input");
		for(var b = 0; b < inputs.length; b++) {
			if(inputs[b].type == "checkbox" && inputs[b].checked == true && inputs[b].className == "styled") {
				inputs[b].previousSibling.style.backgroundPosition = "0 -" + checkboxHeight*2 + "px";
			} else if(inputs[b].type == "checkbox" && inputs[b].className == "styled") {
				inputs[b].previousSibling.style.backgroundPosition = "0 0";
			} else if(inputs[b].type == "radio" && inputs[b].checked == true && inputs[b].className == "styled") {
				inputs[b].previousSibling.style.backgroundPosition = "0 -" + radioHeight*2 + "px";
			} else if(inputs[b].type == "radio" && inputs[b].className == "styled") {
				inputs[b].previousSibling.style.backgroundPosition = "0 0";
			}
		}
	},
	choose: function() {
		option = this.getElementsByTagName("option");
		for(d = 0; d < option.length; d++) {
			if(option[d].selected == true) {
				document.getElementById("select" + this.name).childNodes[0].nodeValue = option[d].childNodes[0].nodeValue;
			}
		}
	}
}
window.onload = Custom.init;

Code (markup):