is-dark-mode.js

const isDarkMode = () => {
  return window.matchMedia("(prefers-color-scheme: dark)").matches;
};

const onThemeChange = (function() {
  let listeners = [];
  let mm = window.matchMedia("(prefers-color-scheme: dark)");
  mm.addEventListener("change", e => {
    for (let listener of listeners) {
      listener(e.matches);
    }
  });
  return callback => {
    listeners.push(callback);
  };
}());