Examine source code of CheapSMS Helper

{
  "manifest_version": 2,
  "name": "CheapSMS Helper",
  "version": "1.4",
  "description": "Extra features for cheapsms.ru",
  "applications": {
    "gecko": {
      "id": "helper@cheapsms.ru"
    }
  },
  "permissions": [
    "storage"
  ],
  "browser_action": {},
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.cheapsms.ru/*"
      ],
      "js": [
        "cheapsms.js"
      ],
      "run_at": "document_end"
    },
    {
      "matches": [
        "*://*.cheapsms.ru/*"
      ],
      "css": [
        "cheapsms.css"
      ],
      "run_at": "document_start"
    }
  ]
}