Change Google Calendar Background

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "manifest_version": 3,
  "name": "__MSG_name__",
  "version": "1.0.4",
  "default_locale": "en",
  "description": "__MSG_description__",
  "permissions": [
    "storage",
    "tabs"
  ],
  "host_permissions": [
    "*://chrome-tool.github.io/*"
  ],
  "action": {
    "default_icon": "assets/icons/icon128.png",
    "default_popup": "popup.html"
  },
  "icons": {
    "16": "assets/icons/icon16.png",
    "48": "assets/icons/icon48.png",
    "128": "assets/icons/icon128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://calendar.google.com/*"
      ],
      "css": [
        "assets/css/style.css"
      ],
      "js": [
        "assets/js/loader.js"
      ]
    }
  ],
  "web_accessible_resources": [
    {
      "resources": [
        "assets/js/content.js",
        "assets/js/utils.js"
      ],
      "matches": [
        "<all_urls>"
      ]
    }
  ]
}