Where is the class "widget_twitter"?

Hello ! 🙂

I try to stylize the widget, but i don’t find the class “widget_twitter” in the wrapper div.
How to get it?

My HTML code (simplified) :

<div id="widget_main">
    <div class="other_widget1">
	<div>...</div>
    <div>
</div>
<div id="widget_main">
    <div class="other_widget2">
		<div>...</div>
    <div>
</div>
<div id="widget_main">
    <div>
        <h2 class="widget-title"><span class='twitterwidget twitterwidget-title'>Title</span></h2>
        <ul>
            <li>
                <span class='entry-content'>The tweet</span>
                <span class='entry-meta'><span class='time-meta'>Time meta</span></span>
            </li>
        </ul>
    <div>
</div>

Thanks in advance.
Nicolas

http://wordpress.org/extend/plugins/twitter-widget-pro/