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

// Toggle theme

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

if (getTheme !== null) {
  document.body.classList.toggle("dark-theme", isDark);
}

themeToggle.addEventListener("click", () => {
  document.body.classList.toggle("dark-theme");
  window.localStorage &&
    window.localStorage.setItem(
      "theme",
      document.body.classList.contains("dark-theme") ? "dark" : "light",
    );
});