Custom CSS for a particular text/html widget in WordPress

From firebug, you can see the ID name of the text widget, which is “text-some number” and this is what we need to customize. the specific WordPress widget has the class name “widget” (from class=”widget widget_text”.). All we need to do is add a new CSS selector to style the selected widget.

#sidebar .widget #text-3564277 {
background-color: #fff;
border: 1px solid #ddd;
padding: 10px;
}

or you can go for any plugins like widget css classes
which will add a field to all wordpress widgets for entering the custom class. then you can style that class.