Examine source code of Days Until

{
  "manifest_version": 2,
  "name": "Days Until",
  "version": "1.2.1",
  "description": "Track multiple countdown timers to upcoming events",
  "browser_specific_settings": {
    "gecko": {
      "id": "{845ceab0-eeb3-4ccf-98fd-5c79f8750f50}",
      "strict_min_version": "96.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "icons": {
    "16": "icons/ico16.png",
    "32": "icons/ico32.png",
    "48": "icons/ico48.png",
    "96": "icons/ico96.png",
    "128": "icons/ico128.png"
  },
  "browser_action": {
    "default_popup": "gui/main.html",
    "default_icon": {
      "16": "icons/ico16.png",
      "32": "icons/ico32.png",
      "48": "icons/ico48.png",
      "96": "icons/ico96.png",
      "128": "icons/ico128.png"
    }
  },
  "background": {
    "scripts": [
      "core/bg.js"
    ]
  },
  "permissions": [
    "storage"
  ]
}