ui/webui/resources/js/dark_mode.js

// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

cr.addWebUIListener('dark-mode-changed', darkMode => {
  loadTimeData.overrideValues({'darkMode': darkMode});
  document.documentElement.toggleAttribute('dark', darkMode);
});

chrome.send('observeDarkMode');