External JS & CSS

.myclass {
	font-family: verdana, sans-serif;
	font-size: 2em;
	background-color: red;
}

<script type="text/javascript">
function changecss(myclass,element,value) {
	var CSSRules
	if (document.all) {
		CSSRules = 'rules'
	}
	else if (document.getElementById) {
		CSSRules = 'cssRules'
	}
	for (var i = 0; i < document.styleSheets.length; i++) {
		for (var j = 0; j < document.styleSheets[i][CSSRules].length; j++) {
			if (document.styleSheets[i][CSSRules][j].selectorText == myclass) {
				document.styleSheets[i][CSSRules][j].style[element] = value
			}
		}	
	}
}

<div onclick="changecss('.myclass','backgroundColor','blue')">changecss</div>
<div class="myclass">
myclass content
</div>

function changeCSS(cssFile)
{
cssLink.href=cssFile;
}