.back-to-top {
    position: fixed;
    bottom: 20px;
    right: 20px;
    display: none;
    color: #fff;
    font-size: 32px;
    z-index: 9999;
    opacity: 0.7;
}

.back-to-top:hover {
    opacity: 1;
}

@media (max-width: 767px) {
    .back-to-top {
        font-size: 24px;
        bottom: 10px;
        right: 10px;
    }
}
$(document).ready(function(){
    $(window).scroll(function(){
        if ($(this).scrollTop() > 100) {
            $('.back-to-top').fadeIn();
        } else {
            $('.back-to-top').fadeOut();
        }
    });

    $('.back-to-top').click(function(){
        $("html, body").animate({ scrollTop: 0 }, 600);
        return false;
    });
});