#button_up{
    position: fixed;
    bottom: 30px;
    display: block;
    left: 30px;
    background: #cccccc7d;
    width: 50px;
    height: 50px;
    opacity: 0;
    transition: all .3s ease-in-out;
}

#button_up img{
    margin: 5px 5px;
    width: 40px;
    opacity: 80%;
}