.overlay {
    display: none;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.8);
    position: fixed;
    z-index: 13;
}
.btn_area {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
    text-align: center;
    z-index: 12;
}
.btn_area button {
    display: block;
    margin: 20px auto 0 auto;
    padding: 10px 30px;
    background: #fff;
    color: #000;
    border: none;
    border-radius: 5px;
}

@media screen and (max-width: 768px){
    .btn_area {width: 80vw;}
    .btn_area .add img{
        width: 100%;
    }
}
@media screen and (min-width: 769px){
    .btn_area .add img{
    width: 600px;
    }
}