Examine source code of Emotions for HN

{
  "name": "Emotions for HN",
  "version": "0.0.1",
  "description": "Add emotions to the HN comments!",
  "manifest_version": 2,
  "permissions": [
    "https://news.ycombinator.com/",
    "https://hn-emotions.appspot.com/",
    "http://localhost:8080/",
    "activeTab"
  ],
  "icons": {
    "48": "icons/hn-emotions-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://news.ycombinator.com/item*"
      ],
      "run_at": "document_idle",
      "css": [
        "css/content.css"
      ],
      "js": [
        "js/content.js"
      ]
    }
  ]
}