Examine source code of Colored buttons

{
  "manifest_version": 2,
  "name": "Colored buttons",
  "version": "1.0.2",
  "description": "Makes buttons colored",
  "applications": {
    "gecko": {
      "id": "ytfastener@youtube.com",
      "strict_min_version": "45.0"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "css": [
        "custom.css"
      ]
    }
  ]
}