Oops solved it myself:
a.textbox-icon.combo-arrow.open {
transform: rotate(180deg);
background-color: pink;
}
var pan = me.combo('panel');
pan.panel({
openAnimation : 'slide',
closeAnimation : 'slide',
onOpen: function(){
me.combo('getIcon', 0).addClass('open');
},
onClose: function(){
me.combo('getIcon', 0).removeClass('open');
}
});