Popup-Close button not round

Popup-Close button not round

I have a popup-closebutton which is called with the class ui-btn-left but it hasn't got the stylish round form.
What is wrong? Any idea?
The html-code:

<div data-role="popup" id="popupaktuell" data-transition="pop" data-overlay-theme="a" data-theme="d" data-corners="true">
<a href="#" data-rel="back" data-role="button" data-theme="a" data-icon="delete" data-iconpos="notext" class="ui-btn-left"></a>

Any help apreciated!