Examine source code of CALCU

{
  "description": "Adds a calculator as an extension, can be used with keys as well as mouse",
  "manifest_version": 2,
  "name": "CALCU",
  "version": "1.0",
  "icons": {
    "48": "icons/calcu.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/calcu.png",
    "theme_icons": [
      {
        "light": "icons/calcu.png",
        "dark": "icons/calcu.png",
        "size": 32
      }
    ],
    "default_title": "CALCU",
    "default_popup": "popup/calcu.html"
  }
}