SO Help: Change DIV background color with .click

HTML

<div id="div4" class="testDIV">Click to increase height</div>

CSS

div {
    background-color: green;     
    text-align: center; 
}

JavaScript

function writeStyles(styleName, cssText) {
    var styleElement = document.getElementById(styleName);
    if (styleElement) document.getElementsByTagName('head')[0].removeChild(
        styleElement);
    styleElement = document.createElement('style');
    styleElement.type = 'text/css';
    styleElement.id = styleName;
    styleElement.innerHTML = cssText;
    document.getElementsByTagName('head')[0].appendChild(styleElement);
}

function changeHeight() {
    var cssText = '.testDIV{ height:' + height + 'px !important; }';
    writeStyles('styles_js', cssText)
}
var height = 50;
$("#div4").on("click", function() {
    height = height + 10;
    changeHeight();
});
changeHeight();