Examine source code of fixedHackerNews

{
  "name": "fixedHackerNews",
  "version": "1.0.0",
  "description": "Fix Hacker News' block formatting",
  "manifest_version": 2,
  "icons": {
    "16": "icons/16.png",
    "48": "icons/48.png",
    "128": "icons/128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://news.ycombinator.com/*"
      ],
      "css": [
        "fixPre.css"
      ]
    }
  ],
  "permissions": [
    "activeTab"
  ]
}