Examine source code of Page Dark

{
  "manifest_version": 2,
  "applications": {
    "gecko": {
      "id": "jid1-KURHKhwWqnpmKQ@jetpack"
    }
  },
  "name": "Page Dark",
  "short_name": "PD",
  "version": "2.1.0",
  "description": "Hides the page content — except videos, games and other media — with a simple black overlay. No fancy extras, just darkness!",
  "author": "marnick.leau@gmail.com",
  "icons": {
    "16": "images/icon-on-16.png",
    "19": "images/icon-on-19.png",
    "38": "images/icon-on-38.png",
    "48": "images/icon-on-48.png",
    "128": "images/icon-on-128.png"
  },
  "options_ui": {
    "page": "options.html",
    "chrome_style": true
  },
  "browser_action": {
    "default_icon": {
      "19": "images/icon-off-19.png",
      "38": "images/icon-off-38.png"
    },
    "default_title": "Toggle darkness"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "permissions": [
    "storage",
    "activeTab",
    "contextMenus"
  ],
  "commands": {
    "darken-page": {
      "suggested_key": {
        "default": "Alt+Shift+P"
      },
      "description": "Darken the current tab."
    }
  }
}