No es más sencillo esto?
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
<script>
function loadCss(css) {
if(document.getElementById('estilos'))
document.getElementsByTagName('head')[0].removeChild(document.getElementById('estilos'));
var x = document.createElement("link");
x.rel="stylesheet";
x.href=css;
x.id="estilos";
document.getElementsByTagName('head')[0].appendChild(x);
}
window.onload=function(){loadCss('estilos_1.css');}
</script>
</head>
<body>
<div id="navcontainer">
<ul id="navlist">
<li id="active"><a href="#" id="current">Item one</a></li>
<li><a href="#">Item two</a></li>
<li><a href="#">Item three</a></li>
<li><a href="#">Item four</a></li>
<li><a href="#">Item five</a></li>
</ul>
</div><br />
<div id="submenu" align="center"><a href="javascript:loadCss('estilos_2.css')">estilo 2</a> <span>|</span> <a href="javascript:loadCss('estilos_1.css')">estilo 1 </a></div>
</body>
</html>