in assets/js/slick.js [969:991]
    Slick.prototype.fadeSlideOut = function(slideIndex) {
        var _ = this;
        if (_.cssTransitions === false) {
            _.$slides.eq(slideIndex).animate({
                opacity: 0,
                zIndex: _.options.zIndex - 2
            }, _.options.speed, _.options.easing);
        } else {
            _.applyTransition(slideIndex);
            _.$slides.eq(slideIndex).css({
                opacity: 0,
                zIndex: _.options.zIndex - 2
            });
        }
    };