Examine source code of Personalized Send Button

{
  "content_scripts": [
    {
      "css": [
        "main.css"
      ],
      "js": [
        "main.js"
      ],
      "matches": [
        "https://mail.google.com/*"
      ]
    }
  ],
  "description": "No more accidental Email to wrong person. Recipient Name and Photo within the \"send\" button, reminds you of your intended recipient",
  "homepage_url": "http://cognisend.com/",
  "icons": {
    "16": "logo1.png",
    "128": "logo.png"
  },
  "manifest_version": 2,
  "name": "Personalized Send Button",
  "permissions": [],
  "version": "20.5.6.921",
  "web_accessible_resources": [
    "lib/*",
    "content.js"
  ]
}