Examine source code of FFN+

{
  "manifest_version": 2,
  "name": "ffn",
  "version": "1.0",
  "description": "Increase default font size and enable copying of story text for commenting purposes.",
  "icons": {
    "48": "icons/ffn.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.fanfiction.net/*"
      ],
      "js": [
        "ffn.js"
      ],
      "run_at": "document_idle"
    }
  ]
}