Examine source code of Ninja Listening Trainer

{
  "manifest_version": 3,
  "name": "Ninja Listening Trainer for YouTube",
  "version": "0.2.6",
  "icons": {
    "16": "pink shuriken 128.png",
    "48": "pink shuriken 128.png",
    "128": "pink shuriken 128.png"
  },
  "description": "Mark a YouTube fragment and replay it through a customizable speed/subtitle sequence for listening and pronunciation practice.",
  "permissions": [
    "storage"
  ],
  "background": {
    "service_worker": "background.js",
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*",
        "file:///*.mp3",
        "file:///*.m4a",
        "file:///*.wav",
        "file:///*.ogg",
        "file:///*.oga",
        "file:///*.flac",
        "file:///*.aac",
        "file:///*.weba"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "content.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "options_page": "options.html",
  "action": {
    "default_title": "Ninja Listening Trainer",
    "default_popup": "popup.html",
    "default_icon": {
      "16": "pink shuriken 128.png",
      "48": "pink shuriken 128.png",
      "128": "pink shuriken 128.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "ninja-listening-trainer-public@kouichi.example",
      "strict_min_version": "140.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "web_accessible_resources": [
    {
      "resources": [
        "pink shuriken 128.png"
      ],
      "matches": [
        "*://*.youtube.com/*",
        "<all_urls>"
      ]
    }
  ]
}