Examine source code of Ecosia Count Trees

{
  "manifest_version": 2,
  "name": "Ecosia Count Trees",
  "version": "1.0",
  "description": "Counts how many trees you have planted while using Ecosia.",
  "icons": {
    "48": "icons/ecosia-tree.svg",
    "96": "icons/ecosia-tree.svg"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.ecosia.org/*"
      ],
      "js": [
        "ecosia_trees.js"
      ]
    }
  ]
}