Examine source code of Journey Themes Pro

{
  "description": "__MSG_extensionDescription__",
  "icons": {
    "48": "icons/logo_48.png",
    "64": "icons/logo_64.png"
  },
  "manifest_version": 2,
  "name": "Hourly Theme Rotation",
  "permissions": [
    "alarms",
    "theme"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "version": "1.0",
  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "91.1.0",
      "id": "{23deaae1-91c9-435d-a0f6-ebec30b78d20}"
    }
  }
}