chrome/browser/resources/ntp4/incognito_tab.js

// Copyright 2017 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.

window.addEventListener('load', function() {
  cr.addWebUIListener('theme-changed', themeData => {
    document.documentElement.setAttribute(
        'hascustombackground', themeData.hasCustomBackground);
    $('incognitothemecss').href =
        'chrome://theme/css/incognito_new_tab_theme.css?' + Date.now();
  });
  chrome.send('observeThemeChanges');
});

// Handle the bookmark bar, theme, and font size change requests
// from the C++ side.
const ntp = {
  /** @param {string} attached */
  setBookmarkBarAttached: function(attached) {
    document.documentElement.setAttribute('bookmarkbarattached', attached);
  },
};