:root {
    --star-background-start: #16161d;
    --star-background-mid: #1f1f3a;
    --star-background-end: #3b2f4a;
}

body {
    background-color: var(--star-background-start);
}

#nen-bau-troi-dem {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient(
        var(--star-background-start),
        var(--star-background-mid),
        var(--star-background-end)
    );
    overflow: hidden;
    pointer-events: none;
    z-index: -1;
}

#nen-bau-troi-dem .stars {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
}

#nen-bau-troi-dem .star {
    animation-name: twinkle;
    animation-timing-function: ease-in-out;
    animation-iteration-count: infinite;
    animation-direction: alternate;
}

@keyframes twinkle {
    0% {
        opacity: 0.2;
    }
    50% {
        opacity: 1;
    }
    100% {
        opacity: 0.2;
    }
}