.btn_up {
    position: fixed;
    bottom: -54px;
    right: 29px;
    /*width: 50px;
    height: 50px;*/
    border-radius: 50%;
    border: 2px solid #ccc;
    background: #333;
    z-index: 100;
    outline: 0!important;
    opacity: 0.5;
    transition: .4s;
}

.btn_up:after {
    content: '';
    width: 10px;
    height: 10px;
    border-top: 2px solid #ccc;
    border-left: 2px solid #ccc;
    transform: rotate(45deg);
    position: absolute;
    top: 4px; bottom: 0;
    left: 0;  right: 0;
    margin: auto;
}

.active_btn_up {
    bottom: 87px;
}

@media (max-width:760px) {
    .btn_up {
        width: 54px;
        height: 54px;
    }
}

@media (min-width:761px) {
    .btn_up {
        width: 54px;
        height: 54px;
    }
}