Examine source code of TwitterBday

{
  "description": "Show upcoming birthdays of your Twitter friends.",
  "manifest_version": 2,
  "name": "TwitterBday",
  "version": "1.0.1",
  "homepage_url": "https://twitterbday.com/",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://twitter.com/*",
        "*://*.twitter.com/*"
      ],
      "js": [
        "twitter.js"
      ],
      "css": [
        "base.css"
      ]
    }
  ],
  "permissions": [
    "storage",
    "webRequest",
    "*://twitter.com/*",
    "*://*.twitter.com/*"
  ],
  "icons": {
    "32": "icon-32.png",
    "36": "icon-36.png",
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "web_accessible_resources": [
    "logo.png",
    "close.png",
    "prev.png",
    "next.png",
    "tbday-collapse.png",
    "profile.png",
    "reload.png",
    "heart.png"
  ]
}