Examine source code of GroupTime

{
  "manifest_version": 2,
  "version": "0.0.4",
  "short_name": "GT",
  "name": "GroupTime Extension",
  "description": "GroupTime Extension",
  "browser_action": {
    "default_title": "GT",
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "extension-mozilla@grouptime.com",
      "strict_min_version": "57.0"
    }
  },
  "background": {
    "scripts": [
      "background.bundle.js"
    ]
  },
  "content_scripts": [
    {
      "css": [
        "yourcss.css"
      ],
      "matches": [
        "*://mail.google.com/*"
      ],
      "run_at": "document_end",
      "js": [
        "contentScript.bundle.js"
      ]
    }
  ],
  "externally_connectable": {
    "matches": [
      "*://mail.google.com/*"
    ]
  },
  "permissions": [
    "tabs",
    "storage",
    "identity",
    "webRequest",
    "https://develop.grouptime.io/api*"
  ],
  "web_accessible_resources": [
    "yourcss.css",
    "fonts/Inter-VariableFont_slnt,wght.ttf",
    "contentScript.bundle.js",
    "img/Originalbutton.png"
  ],
  "icons": {
    "16": "img/icon-16.png",
    "48": "img/icon-48.png",
    "128": "img/icon-128.png"
  }
}