Examine source code of Let It Snow

{
  "manifest_version": 2,
  "name": "Let It Snow",
  "developer": {
    "name": "Jared Krause",
    "url": "https://kravse.dev"
  },
  "version": "0.1.3",
  "description": "Add cute snowflakes to your browser on every page.",
  "icons": {
    "16": "images/icon16.png",
    "48": "images/icon.png",
    "128": "images/icon128.png"
  },
  "permissions": [
    "storage",
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "images/icon.png",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "run_at": "document_idle",
      "js": [
        "assets/christmas.js"
      ],
      "css": [
        "assets/main.css"
      ]
    }
  ]
}