Examine source code of Invert Image color

{
  "manifest_version": 2,
  "name": "Invert Image",
  "version": "3.0",
  "description": "Inverts color of an image/(any part of a page) to black for night time reading",
  "homepage_url": "https://github.com/vycb/InvertImage",
  "icons": {
    "48": "icons/icon-48.png",
    "64": "icons/icon-64.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "main.js"
      ],
      "all_frames": true
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "contextMenus",
    "activeTab"
  ]
}