.middle {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);

}

html,
body {
    height: 100%;
    background: #3f51b5;
}

.box {
    width: 40%;
}

@media only screen and (max-width: 992px) {
    .box {
        width: 100%;
        height: 100%;
    }
}