Just copy and paste the code below into your Body of your site.

// Select fade-effect below:
// Set 1 if the background may fade from dark to medium 
// Set 2 if the background may fade from light to medium 
// Set 3 if the background may fade from very dark to very light light
// Set 4 if the background may fade from light to very light
// Set 5 if the background may fade from dark to very dark 
var fade_effect=3

// What type of gradient should be applied Internet Explorer 5x or higher?
// Set "none" or "horizontal" or "vertical"
var gradient_effect="horizontal"

// Speed higher=slower
var speed=60

///////////////////////////////////////////////////////////////////////////
// CONFIGURATION ENDS HERE
///////////////////////////////////////////////////////////////////////////

var browserinfos=navigator.userAgent 
var ie4=document.all&&!document.getElementById
var ie5=document.all&&document.getElementById&&!browserinfos.match(/Opera/)
var ns4=document.layers
var ns6=document.getElementById&&!document.all
var opera=browserinfos.match(/Opera/)  
var browserok=ie4||ie5||ns4||ns6||opera

if (fade_effect==1) {
	var darkmax=1
	var lightmax=127
}
if (fade_effect==2) {
	var darkmax=127
	var lightmax=254
}
if (fade_effect==3) {
	var darkmax=1
	var lightmax=254
}
if (fade_effect==4) {
	var darkmax=190
	var lightmax=254
}
if (fade_effect==5) {
	var darkmax=1
	var lightmax=80
}
var hexc = new Array('0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F')

var newred
var newgreen
var newblue
var oldred
var oldgreen
var oldblue

var redcol_1
var redcol_2 
var greencol_1 
var greencol_2 
var bluecol_1 
var bluecol_2 
var oldcolor
var newcolor
var firsttime=true
 
var stepred=1
var stepgreen=1
var stepblue=1

function setrandomcolor() {
	var range=(lightmax-darkmax)
	if (firsttime) {
		newred=Math.ceil(range*Math.random())+darkmax
		newgreen=Math.ceil(range*Math.random())+darkmax
		newblue=Math.ceil(range*Math.random())+darkmax
		firsttime=false
	}
	
	oldred=Math.ceil(range*Math.random())+darkmax
	oldgreen=Math.ceil(range*Math.random())+darkmax
	oldblue=Math.ceil(range*Math.random())+darkmax
	
	stepred=newred-oldred
	if (oldred>newred) {stepred=1}
	else if (oldred<newred) {stepred=-1}
	else {stepred=0}
	
	stepgreen=newgreen-oldgreen
	if (oldgreen>newgreen) {stepgreen=1}
	else if (oldgreen<newgreen) {stepgreen=-1}
	else {stepgreen=0}
	
	stepblue=newblue-oldblue
	if (oldblue>newblue) {stepblue=1}
	else if (oldblue<newblue) {stepblue=-1}
	else {stepblue=0}
	fadebg()
}

if (browserok) {
	window.onload=setrandomcolor
}
</script>