ghfav.js

function setFavicon(scheme) {
  const svgFavicon = document.querySelector('.js-site-favicon[type="image/svg+xml"]'),
        pngFavicon = document.querySelector('.js-site-favicon[type="image/png"]');

  scheme || (scheme = "light");

  if (svgFavicon && pngFavicon) {
    svgFavicon.href = `favicon-${scheme}.svg`;
    pngFavicon.href = `favicon-${scheme}.png`;
  }
}

function prefersDark() {
  return window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches;
}

prefersDark() && setFavicon("dark");

window.matchMedia("(prefers-color-scheme: dark)").addListener(() => {
  setFavicon(prefersDark() ? "dark" : "light");
})