custom mobile menu (Flatiron)

Try out this Custom CSS. Just change the degrees to rotate the symbol however much you want:

@media screen and (max-width:640px) {
    #mobile-navigation #mobile-navigation-label:after, #mobile-navigation.sqs-mobile-nav-open #mobile-navigation-label:after {
        content: "+";
        font-size: 50px;
        display: inline-block;
    }
    #mobile-navigation:active #mobile-navigation-label:after, #mobile-navigation.sqs-mobile-nav-open #mobile-navigation-label:after {
        -webkit-transform: rotate(50deg);
        -moz-transform: rotate(50deg);
        -o-transform: rotate(50deg);
        -ms-transform: rotate(50deg);
        transform: rotate(50deg);
    }
}

You'll see that you need to set display to block or inline-block because psuedo elements (:after) are inline by default and won't rotate without display:block or display:inline-block.