.jjs-wrap-login {
    position:absolute;
    left:50%;
    top:50%;
    width:500px;
    height:300px;
    margin:-150px 0 0 -250px;
    background-color:#bbb;
    padding:20px;
}

@media all and (max-width: 639px) {
    .jjs-wrap-login {
        position:relative;
        left:auto;
        top:auto;
        width:100%;
        height:auto;
        margin:0;
        background-color:#bbb;
        padding:20px;
    }
}