body, html {
    height: 100%;
    margin: 0;
    padding: 0;
}

.masthead {
    background: url('img/bg-masthead.jpg') no-repeat center center fixed;
    background-size: cover;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

.container.text-center.my-auto {
    flex: 1;
    display: flex;
    flex-direction: column;
}

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