Setting the theme of a checkbox on page load?

Setting the theme of a checkbox on page load?

Looking at the documentation it states that you can set the theme of a checkbox using;

$("input[type='checkbox']").checkboxradio({ theme: "a" });

However when I try and run this code inside the document.ready function it doesn't set the theme? 
I can set a buttons theme using;


$('a').buttonMarkup({ theme: "a" });

Does anyone have any idea what I'm doing wrong?

Thanks.

P.S. I've tried adding thsi as well which didn't help 










$("input[type='checkbox']").checkboxradio('refresh');