hugo-theme-hello-friend/assets/js/theme.js

// Toggle theme

const localTheme = window.localStorage && window.localStorage.getItem("theme");
const themeToggle = document.querySelector(".theme-toggle");

if (localTheme) {
  document.body.classList.remove("light-theme", "dark-theme");
  document.body.classList.add(localTheme);
}

themeToggle.addEventListener("click", () => {
  const themeUndefined = !new RegExp("(dark|light)-theme").test(document.body.className);
  const isOSDark = window.matchMedia("(prefers-color-scheme: dark)").matches;

  if (themeUndefined) {
    if (isOSDark) {
      document.body.classList.add("light-theme");
    } else {
      document.body.classList.add("dark-theme");
    }
  } else {
    document.body.classList.toggle("light-theme");
    document.body.classList.toggle("dark-theme");
  }

  window.localStorage &&
    window.localStorage.setItem(
      "theme",
      document.body.classList.contains("dark-theme") ? "dark-theme" : "light-theme",
    );
});