dark-inline-snippet.js

(() => {
  function setDark(dark) {
    document.documentElement.classList.toggle("dark", dark);
  }
  const mql = window.matchMedia("(prefers-color-scheme: dark)");
  function updateDark() {
    const fromLocalStorage = localStorage.getItem("theme");
    if (fromLocalStorage != null) {
      setDark(fromLocalStorage === "dark");
      return;
    }
    setDark(mql.matches);
  }
  mql.addEventListener("change", updateDark);
  window.addEventListener("storage", updateDark);
  updateDark();
})();