How can I add an extra classname to a widget container in the sidebar?

Question

I would like to add an extra custom classname to the widgetcontainer in the sitebar:

 

Situation now:

<div class="qa-widgets-side qa-widgets-side-low">

<div class="qa-widget-side qa-widget-side-low">

[widget content]

 

</div>

</div>
 

Needed:

<div class="qa-widgets-side qa-widgets-side-low customclassname">

<div class="qa-widget-side qa-widget-side-low">

[widget content]

 

</div>

</div>
 

How can I achieve this?