Examine source code of Screen Shutter

{
  "manifest_version": 2,
  "name": "Screen Shutter",
  "version": "1.0",
  "description": "An extension to hide the contents of the page you're currently viewing.",
  "icons": {
    "96": "icons/twotone_visibility_off_white_48dp.png"
  },
  "permissions": [
    "activeTab",
    "tabs",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "screenshutter.js"
      ]
    }
  ],
  "commands": {
    "toggle-feature": {
      "suggested_key": {
        "default": "Ctrl+Shift+Up"
      },
      "description": "Toggle the screen overlay"
    }
  },
  "browser_action": {
    "default_icon": "icons/twotone_visibility_off_white_48dp.png",
    "default_title": "Toggle Hide Screen"
  },
  "applications": {
    "gecko": {
      "id": "screenshutter@erstwhile.me"
    }
  }
}