Examine source code of BEASTY

{
  "description": "Adds a browser action icon to the toolbar. Click the button to choose a beast. The active tab's body content is then replaced with a picture of the chosen beast. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#beastify",
  "manifest_version": 2,
  "name": "Beastify",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/beastify",
  "icons": {
    "48": "icons/beasts-48.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/beasts-32.png",
    "theme_icons": [
      {
        "light": "icons/beasts-32-light.png",
        "dark": "icons/beasts-32.png",
        "size": 32
      }
    ],
    "default_title": "Beastify",
    "default_popup": "popup/choose_beast.html"
  },
  "web_accessible_resources": [
    "beasts/*.jpg"
  ]
}