Examine source code of LibriVox Playmaker

{
  "manifest_version": 2,
  "name": "LibriVox Playmaker",
  "short_name": "Playmaker",
  "version": "1.0.2",
  "developer": {
    "name": "quartertone",
    "url": "https://vox.quartertone.net/"
  },
  "description": "You can use this bookmarklet on a book/project page of LibriVox (for example). The script will replace the \"Play\" button of each track with an audio element loader. Clicking \"load\" will load that particular audio track in to an HTML audio element so that you can actually play the audio without leaving the page. * After each track finishes playing, the next track with automatically load and start to play. * Note: Some browsers may not autoplay while in the background, and some browsers may not allow autoplay at all. * Browser will remeber your progress in each track you play. * Progress will clear when the track finishes playing. * Browser will remember the last track you played, and will pick up where you left off.",
  "icons": {
    "48": "img/playmaker_white.svg",
    "96": "img/playmaker_white.svg"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "playmaker@vox.quartertone.net"
    }
  },
  "browser_action": {
    "theme_icons": [
      {
        "light": "img/playmaker_white.svg",
        "dark": "img/playmaker_black.svg",
        "size": 48
      },
      {
        "light": "img/playmaker_white.svg",
        "dark": "img/playmaker_black.svg",
        "size": 96
      }
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://librivox.org/*"
      ],
      "exclude_matches": [
        "*://librivox.org/search*",
        "*://librivox.org/reader*",
        "*://librivox.org/"
      ],
      "js": [
        "playmaker.js",
        "action.js"
      ]
    }
  ]
}