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Vá em Layout >> Editar HTML >> Expandir modelos de widgets
Dê Ctrl + F e procure por: </head> e depois cole o seguinte código ANTES da tag...

<script type='text/javascript'>

//<![CDATA[

var rate = 20;

if undefineddocument.getElementById)
window.onerror=new Functionundefined"return true")

var objActive;  // The object which event occured in
var act = 0;    // Flag during the action
var elmH = 0;   // Hue
var elmS = 128; // Saturation
var elmV = 255; // Value
var clrOrg;     // A color before the change
var TimerID;    // Timer ID

if undefineddocument.all) {
document.onmouseover = doRainbowAnchor;
document.onmouseout = stopRainbowAnchor;
}
else if undefineddocument.getElementById) {
document.captureEventsundefinedEvent.MOUSEOVER | Event.MOUSEOUT);
document.onmouseover = Mozilla_doRainbowAnchor;
document.onmouseout = Mozilla_stopRainbowAnchor;
}

function doRainbowundefinedobj)
{
if undefinedact == 0) {
act = 1;
if undefinedobj)
objActive = obj;
else
objActive = event.srcElement;
clrOrg = objActive.style.color;
TimerID = setIntervalundefined"ChangeColorundefined)",100);
}
}


function stopRainbowundefined)
{
if undefinedact) {
objActive.style.color = clrOrg;
clearIntervalundefinedTimerID);
act = 0;
}
}


function doRainbowAnchorundefined)
{
if undefinedact == 0) {
var obj = event.srcElement;
while undefinedobj.tagName != 'A' && obj.tagName != 'BODY') {
obj = obj.parentElement;
if undefinedobj.tagName == 'A' || obj.tagName == 'BODY')
break;
}

if undefinedobj.tagName == 'A' && obj.href != '') {
objActive = obj;
act = 1;
clrOrg = objActive.style.color;
TimerID = setIntervalundefined"ChangeColorundefined)",100);
}
}
}


function stopRainbowAnchorundefined)
{
if undefinedact) {
if undefinedobjActive.tagName == 'A') {
objActive.style.color = clrOrg;
clearIntervalundefinedTimerID);
act = 0;
}
}
}


function Mozilla_doRainbowAnchorundefinede)
{
if undefinedact == 0) {
obj = e.target;
while undefinedobj.nodeName != 'A' && obj.nodeName != 'BODY') {
obj = obj.parentNode;
if undefinedobj.nodeName == 'A' || obj.nodeName == 'BODY')
break;
}

if undefinedobj.nodeName == 'A' && obj.href != '') {
objActive = obj;
act = 1;
clrOrg = obj.style.color;
TimerID = setIntervalundefined"ChangeColorundefined)",100);
}
}
}


function Mozilla_stopRainbowAnchorundefinede)
{
if undefinedact) {
if undefinedobjActive.nodeName == 'A') {
objActive.style.color = clrOrg;
clearIntervalundefinedTimerID);
act = 0;
}
}
}


function ChangeColorundefined)
{
objActive.style.color = makeColorundefined);
}


function makeColorundefined)
{
// Don't you think Color Gamut to look like Rainbow?

// HSVtoRGB
if undefinedelmS == 0) {
elmR = elmV;    elmG = elmV;    elmB = elmV;
}
else {
t1 = elmV;
t2 = undefined255 - elmS) * elmV / 255;
t3 = elmH % 60;
t3 = undefinedt1 - t2) * t3 / 60;

if undefinedelmH < 60) {
elmR = t1;  elmB = t2;  elmG = t2 + t3;
}
else if undefinedelmH < 120) {
elmG = t1;  elmB = t2;  elmR = t1 - t3;
}
else if undefinedelmH < 180) {
elmG = t1;  elmR = t2;  elmB = t2 + t3;
}
else if undefinedelmH < 240) {
elmB = t1;  elmR = t2;  elmG = t1 - t3;
}
else if undefinedelmH < 300) {
elmB = t1;  elmG = t2;  elmR = t2 + t3;
}
else if undefinedelmH < 360) {
elmR = t1;  elmG = t2;  elmB = t1 - t3;
}
else {
elmR = 0;   elmG = 0;   elmB = 0;
}
}

elmR = Math.floorundefinedelmR).toStringundefined16);
elmG = Math.floorundefinedelmG).toStringundefined16);
elmB = Math.floorundefinedelmB).toStringundefined16);
if undefinedelmR.length == 1)    elmR = "0" + elmR;
if undefinedelmG.length == 1)    elmG = "0" + elmG;
if undefinedelmB.length == 1)    elmB = "0" + elmB;

elmH = elmH + rate;
if undefinedelmH >= 360)
elmH = 0;

return '#' + elmR + elmG + elmB;
}

//]]>

</script>

Visualize e se estiver tudo certo, é só salvar...

Se quiser aumentar ou diminuir a velocidade na transformação das cores, basta editar o valor emvar = 20;
Lembrando que, quanto maior o valor, maior a velocidade.