隐私模式下修改标签页标题及自定义新标签页扩展的生效问题求助

我想给新开的隐私标签页设置一个自定义的着陆页面，之前已经靠自定义扩展搞定了普通标签页的效果，但隐私窗口/标签页里就是没法生效，求各位帮忙看看问题出在哪！

我用到的扩展manifest.json代码是这样的：

{
  "manifest_version": 3,
  "name": "Black Background New Tab",
  "version": "1.0",
  "description": "Changes the background color of new tabs to black.",
  "permissions": ["tabs", "activeTab", "storage"],
  "chrome_url_overrides": {
    "newtab": "newtab.html"
  },
  "host_permissions": ["<all_urls>"],
  "incognito": "spanning"
}

对应的newtab.html内容如下：

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>────────────────────────────────────────────</title>
  <style>
    <!-- some style here -->
  </style>
</head>
<body>
  <!-- some code here -->
</body>
</html>

备注：内容来源于stack exchange，提问作者Laplace'sDemon