Examine source code of apply-css

{
  "description": "Adds a page action to toggle applying CSS to pages.",
  "manifest_version": 2,
  "name": "apply-css",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/apply-css",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "page_action": {
    "default_icon": "icons/off.svg",
    "browser_style": true
  },
  "permissions": [
    "activeTab",
    "tabs",
    "<all_urls>"
  ]
}