.slider-arrow {
    position: absolute;
    top: 154px;
    height: 50px;
    width: 50px;
    margin-top: -25px; /* 高さの半分だけネガティブマージン */
    color: black;
    background: transparent;
    opacity: 0.6;
    line-height: 50px;
    font-size: 60px;
    cursor: pointer;
    z-index: 10; /* 重要 */
    text-align: center;
    border-radius: 50%;
}

.slider-prev {
    left: -20px;
}

.slider-next {
    right: -20px;
}

.slider-prev:before {
    margin-right: 40px;
}

.slider-next:before {
    margin-left: 20px;
}
@media screen and (max-width:991px){
    .slider-arrow {
        display: none!important;
    }
}