.fa-chevron-down{ background: url(images/heise.png) no-repeat;width: 16px;height: 16px;}
.accordion li.open i.fa-chevron-down{background: url(images/baise.png) no-repeat;width: 16px;height: 16px;

-webkit-transform: rotate(360deg);
    -ms-transform: rotate(360deg);
    -o-transform: rotate(360deg);
    transform: rotate(360deg);}