Examine source code of Subscription Score

{
  "name": "Subscription Score",
  "description": "Never regret subscribing to a mailing list again",
  "version": "0.4.4",
  "background": {
    "page": "background.html"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "assets/logo.png"
  },
  "icons": {
    "16": "assets/icon-16.png",
    "32": "assets/icon-32.png",
    "64": "assets/icon-64.png",
    "128": "assets/icon.png"
  },
  "web_accessible_resources": [
    "onload.bundle.js",
    "frame.html",
    "frame.bundle.js",
    "frame.css"
  ],
  "permissions": [
    "tabs",
    "storage"
  ],
  "optional_permissions": [
    "<all_urls>"
  ],
  "manifest_version": 2,
  "content_security_policy": "script-src 'self' 'unsafe-eval'; object-src 'self'",
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*/*"
      ],
      "js": [
        "/gmail.bundle.js"
      ],
      "css": [
        "/gmail.bundle.css"
      ],
      "run_at": "document_idle"
    },
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "/content.bundle.js"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "hello@subscriptionscore.com"
    }
  },
  "options_ui": {
    "page": "options.html",
    "open_in_tab": true
  }
}