html,
body {
    height: 100%;
}

header {
    height: 100%;
    display: grid;
    place-content: center;
}

header > div {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center; /* Vertical center of image & text */
}

.logo {
    width: 50%;
}