You need to define editable css in the style declaration block. You can put any html where you want your client to be able to edit.
Example:
(copy and save in the template editor and open in the editor of your campaign).
<html>
<head>
<style type="text/css">
#table{
background-color:#888888;
}
</style>
</head>
<body>
<table id="table">
<tr>
<td>
<div mc:edit="content">
Lorem ipsum
</div>
</td>
</tr>
</table>
</body>
</html>
, - mc: edit.
: http://templates.mailchimp.com/getting-started/template-language/