Examine source code of Good Old Days

{
  "description": "Makes every Website look old with a stylish b&w filter.",
  "manifest_version": 2,
  "name": "Good Old Days",
  "version": "1.1resigned1",
  "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"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{750a909d-62d4-4fea-984e-2a1ee48563e1}"
    }
  }
}