:root{
  --space:#07080d;
  --star1:rgba(255,255,255,.9);
  --star2:rgba(255,255,255,.6);
  --star3:rgba(255,255,255,.35);
}

/* Falls du Dark erzwingen willst: nimm die Media-Query raus und setz color-scheme: dark */
@media (prefers-color-scheme: dark) {

  html {
    /* Vollflächig am Viewport statt am zentrierten Body */
    background-color: var(--space) !important;
    background-image:
      radial-gradient(1px 1px at 20% 30%, var(--star1) 50%, transparent 51%),
      radial-gradient(1px 1px at 80% 60%, var(--star1) 50%, transparent 51%),
      radial-gradient(1px 1px at 40% 70%, var(--star2) 50%, transparent 51%),
      radial-gradient(1px 1px at 65% 20%, var(--star2) 50%, transparent 51%),
      radial-gradient(1.5px 1.5px at 10% 50%, var(--star3) 50%, transparent 51%);
    background-size:
      200px 200px,
      300px 300px,
      150px 150px,
      250px 250px,
      350px 350px;
    background-repeat: repeat;
    background-position:
      0 0, 0 0, 0 0, 0 0, 0 0;

    animation: star-drift 40s linear infinite;
  }

  /* Body evtl. schmal: Hintergrund dort abschalten */
  body { background: transparent !important; }

  html, body { min-height: 100%; }

  @keyframes star-drift {
    from { background-position:
      0 0, 0 0, 0 0, 0 0, 0 0; }
    to   { background-position:
      60px 30px,
      -45px 20px,
      30px -20px,
      -24px -18px,
      18px 36px; }
  }
}