how to change the default class for label in <s:textfield="" label=""/>

[Jibin Najeeb]

The following code
<s:form>
<s:textfield name="username" label="Username"/>
</s:form>

generates the follwing html code

<table>
<tr>
<td class="tdLabel"><label for="login_username" class="label">Username:</label></td>
<td><input type="text" name="username" value="" id="login_username"/>
</td>
</tr>
</table>

I want to change the value of label class--- class="label" which is default. Is there any method with default theme as xhtml..