html,
body {
    padding: 0;
    margin: 0;
    height: 100%;
    width: 100%;
    background: url(https://images.unsplash.com/photo-1544829728-e5cb9eedc20e?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1500&q=80);
    background-size: cover;
}

form {
    position: absolute;
    width: 30%;
    left: 50%;
    top: 50%;
    -webkit-transform: translate(-50%, -50%);
    transform: translate(-50%, -50%);
    padding: 20px;
    border-radius: 10px;
    background: white;
}

input,
button,
select {
    width: 100%;
}

@media only screen and (max-width: 600px) {
    form {
        width: calc(100% - 50px);
    }
}