Examine source code of Better Reader

{
  "name": "Better Reader",
  "version": "0.5",
  "description": "Better Reading styles for websites. First release include styles for madhubanmurli.org",
  "manifest_version": 2,
  "icons": {
    "16": "icons/logo16.png",
    "64": "icons/logo64.png",
    "128": "icons/logo128.png"
  },
  "browser_action": {
    "default_icon": "icons/logo128.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "storage",
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://madhubanmurli.org/*"
      ],
      "js": [
        "betterReader.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "styles/sites/madhubanmurli/sepia.css",
    "styles/sites/madhubanmurli/sepia_selection.css",
    "styles/sites/madhubanmurli/dark.css"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "br@exploremill.com",
      "strict_min_version": "57.0"
    }
  }
}