/*If you want a css only solution you can use active*/
.crossRotate:active {
   transform: rotate(45deg);
   -webkit-transform: rotate(45deg);
   -ms-transform: rotate(45deg);
}

/*But the transformation will not persist when the activity moves. 
For that you need javascript (jquery click and css is the cleanest IMO).*/

<script>
$( ".crossRotate" ).click(function() {
    if (  $( this ).css( "transform" ) == 'none' ){
        $(this).css("transform","rotate(45deg)");
    } else {
        $(this).css("transform","" );
    }
});
</script>