core/static/base.css

html,
body {
    height: 100%;
}

body {
    display: flex;
    flex-direction: column;
}

main {
    flex: 1;
}

#loader {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(255, 255, 255, 0.8);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 9999;
}

.spinner-border {
    width: 3rem;
    height: 3rem;
}