It has been long time after question but;
You don't need a CSS file for GeoExt2. It is enough to show GeoExt2-loader.js directory in your index.html file. For example my app like this
<html>
<head>
<title>MY APP</title>
<link rel="stylesheet" type="text/css" href="ext41/resources/css/ext-all-gray.css">
<script type="text/javascript" src="ext41/ext-all.js"></script>
<script type="text/javascript" src="ext41/locale/ext-lang-tr.js"></script>
<script type="text/javascript" src="ol/OpenLayers.js"></script>
<script type="text/javascript" src="GeoExt2/.../loader.js"></script>
<script type="text/javascript" src="myapp.js"></script>
<!-- other js files -->
</head>
<body>
</body>
</html>
And finally you can need to modify your loader.js file to show Ext JS 4 and GeoExt2 directory.
AN IMPORTANT NOTE: GeoExt2 works with only ExtJS 4.1.x and 4.0.x not ExtJS 4.2.x