cant´t change icon

cant´t change icon

Hi,
I got some scripts from a former colleague to work on and have a problem:

I´m looking for a way to change the symbol on a select form field.
Usually it´s a filled triangle pointing down, but all I get is a grey, filled circle instead

I´ve tried it with:
  1. $( "#XYZ" ).selectmenu({ icons: { button: "ui-icon-triangle-1-s" } });
where XYZ is the id of the <select>

the statement is called inside
  1. $(function(){});
Thanks for your help