Examine source code of Gmail Sender Icons Pro

{
  "manifest_version": 2,
  "name": "Gmail Sender Icons",
  "author": "Digital Inspiration",
  "homepage_url": "https://digitalinspiration.com/",
  "description": "Quickly identify the sender of email messages in Gmail without opening the message.",
  "version": "4.0",
  "permissions": [
    "storage",
    "https://mail.google.com/"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ],
      "run_at": "document_end"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "web_accessible_resources": [
    "*.map"
  ],
  "short_name": "GmailSenderIcons",
  "browser_action": {
    "default_icon": "icon.png",
    "default_title": "Show sender icons in Gmail",
    "default_popup": "settings.html"
  },
  "icons": {
    "32": "icon.png",
    "64": "icon.png",
    "128": "icon.png",
    "256": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{1e761571-44b4-498b-908b-be64a22da310}"
    }
  }
}