Examine source code of Dynamic Theme

{
  "manifest_version": 2,
  "description": "Dynamically set the firefox theme",
  "version": "1.0",
  "name": "Dynamic Theme",
  "permissions": [
    "theme",
    "nativeMessaging"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "dynamicTheme@bassed.ca"
    }
  }
}