Specify CSS class in modal confirmation form

A want to apply come CSS statements for confirmation form (in modal popup).
But there is no class specified or something similar to get hooked.

<div tabindex="-1" role="dialog" class="ui-dialog ui-corner-all ui-widget ui-widget-content ui-front ui-dialog-buttons"
     aria-describedby="drupal-modal"
     style="position: fixed; height: auto; width: 300px; top: 155.79px; left: 9.22745px; z-index: 101;"
     aria-labelledby="ui-id-11">
    <div class="ui-dialog-titlebar ui-corner-all ui-widget-header ui-helper-clearfix">
        <span id="ui-id-11"
              class="ui-dialog-title">Process transition?</span>
        <button type="button" class="ui-dialog-titlebar-close"></button>
    </div>
    <div id="drupal-modal" class="ui-front ui-dialog-content ui-widget-content"
         style="width: auto; min-height: 17.604px; max-height: 426.604px; height: auto;">
    </div>
</div>