.sessionlogin-page {
    height: 100vh;
    overflow: hidden;
    display: flex;
    align-items: center; /* vertical centering */
    justify-content: center; /* horizontal centering */
    background: url('/images/bg-auth-overlay.png') center/cover no-repeat;
}

    /* then let the inner .container scroll if it ever overflows,
     or remove margin-top since we’re centering by flex */
    .sessionlogin-page .container {
        margin-top: 0 !important;
    }