You need to add the class="claro" to the body tag. <body class="claro" style="font-family: Arial Unicode MS,Arial,sans-serif; padding: 10px;">
However you might want to look into use the compact build instead since you are working with ExtJS. http://help.arcgis.com/EN/webapi/javascript/arcgis/help/jshelp_start.htm#jshelp/inside_compactbuild....