Examine source code of Simple Close Tab button

{
  "manifest_version": 2,
  "name": "Simple Close Tab button",
  "version": "1.3resigned1",
  "description": "Adds a Close Tab button to the toolbar.",
  "icons": {
    "16": "icons/Close.svg",
    "24": "icons/Close.svg",
    "32": "icons/Close.svg",
    "48": "icons/Close.svg",
    "64": "icons/Close.svg",
    "96": "icons/Close.svg"
  },
  "applications": {
    "gecko": {
      "strict_min_version": "55.0",
      "id": "{fe8cab45-aaee-45f8-8765-781e0ebea300}"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_title": "Close tab",
    "browser_style": true,
    "default_icon": {
      "16": "icons/Close-dark.svg",
      "24": "icons/Close-dark.svg",
      "32": "icons/Close-dark.svg",
      "48": "icons/Close-dark.svg",
      "64": "icons/Close-dark.svg",
      "96": "icons/Close-dark.svg"
    },
    "theme_icons": [
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 16
      },
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 24
      },
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 32
      },
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 48
      },
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 64
      },
      {
        "light": "icons/Close-light.svg",
        "dark": "icons/Close-dark.svg",
        "size": 96
      }
    ]
  }
}