Examine source code of tweakslack

{
  "name": "tweakslack",
  "version": "1.2.0",
  "manifest_version": 2,
  "description": "Tweak Slack UI",
  "icons": {
    "48": "icons/48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://app.slack.com/*",
        "https://*.slack.com/archives/*"
      ],
      "js": [
        "page.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e730fdac-23a4-4715-a16d-ca0f3bb3da11}"
    }
  }
}