Your initial thought was probably correct: this type of thing happens when you don't properly include the appropriate theme's stylesheet or you don't assign the proper class to the body of your page. Make sure your page has the following ifin your head tag:
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/2.3/js/dojo/dijit/themes/tundra/tundra.css">
and
<body class="tundra">