blitzkit/packages/website/src/components/EasterEggs.astro

<script>
  import { isHalloween } from "../core/blitzkit/isHalloween";

  const HALLOWEEN_SATURATION_DELTA = 0.35;
  const HALLOWEEN_SATURATION_TAU = 0.5;
  const HALLOWEEN_BRIGHTNESS_DELTA = 0.1;
  const HALLOWEEN_BRIGHTNESS_TAU = 7;

  function halloweenFrame() {
    const t = Date.now() / 1000;

    document.body.style.filter = `saturate(${
      1 -
      HALLOWEEN_SATURATION_DELTA *
        (1 + (0.5 + 0.5 * Math.sin(HALLOWEEN_SATURATION_TAU * t)))
    }) brightness(${
      1 -
      HALLOWEEN_BRIGHTNESS_DELTA *
        (1 + (0.5 + 0.5 * Math.sin(HALLOWEEN_BRIGHTNESS_TAU * t)))
    })`;
  }

  if (isHalloween()) {
    document.body.classList.add("halloween");
    setInterval(halloweenFrame, 1000 / 30);
  }
</script>