:root {
  color-scheme: light dark;
  --bg: #fff;
  --fg: #000;
  --bg: light-dark(#fff, #000);
  --fg: light-dark(#000, #fff);
  background: var(--bg);
}

:root::before {
  content: "";
  position: absolute;
  font-size: 1vmin;
  top: 50%;
  left: 50%;
  translate: -50% -50%;
  width: 50em;
  aspect-ratio: 1;
  border-top: 2em solid var(--fg);
  background: linear-gradient(90deg, #0000 calc(50% - 1em), var(--fg) 0 calc(50% + 1em), #0000 0);
  box-sizing: content-box;
}