Examine source code of Screen Clock

{
  "manifest_version": 2,
  "name": "Screen Clock",
  "version": "1.0",
  "description": "A clock shows week day, time and date.",
  "content_scripts": [
    {
      "matches": [
        "*://*.mozilla.org/*"
      ],
      "html": [
        "index.html"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ca985d2a-f191-4652-a9bd-66119a1e7910}"
    }
  }
}