Examine source code of Scalegray

{
  "manifest_version": 2,
  "name": "Scalegray",
  "version": "0.7.0",
  "description": "Make distracting sites grayscale",
  "homepage_url": "https://github.com/semaj/firefox-grayscale",
  "icons": {
    "48": "icons/icon-dark.svg"
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{64ebe59a-960d-4cb8-8eaf-55c50ad8e05f}",
      "strict_min_version": "42.0"
    }
  },
  "browser_action": {
    "default_icon": "icons/icon-dark.svg",
    "theme_icons": [
      {
        "light": "icons/icon-dark.svg",
        "dark": "icons/icon-light.svg",
        "size": 19
      }
    ],
    "default_popup": "popup/popup.html",
    "default_title": "Scalegray"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content-script.js"
      ],
      "run_at": "document_end"
    },
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content-script.js"
      ],
      "run_at": "document_start"
    }
  ],
  "options_ui": {
    "browser_style": true,
    "page": "options/options.html",
    "open_in_tab": false
  }
}