Examine source code of Pic Style - Checkerboard

{
  "applications": {
    "gecko": {
      "id": "picstyle@mythweb.ro",
      "strict_min_version": "54.0"
    }
  },
  "manifest_version": 2,
  "name": "Pic Style - Checkerboard",
  "author": "Coyotee",
  "version": "2.2resigned1",
  "description": "Render standalone transparent images with a checkerboard background!",
  "icons": {
    "64": "appearance-64.png"
  },
  "permissions": [
    "tabs",
    "file:///*",
    "*://*/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "file:///*",
        "*://*/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ]
}