Examine source code of Halloween mixed

{
  "description": "Theme using multiple additional_backgrounds images. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#themes",
  "manifest_version": 2,
  "name": "Halloween mixed",
  "version": "5.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/themes/",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "pum.png",
        "pum-left.png"
      ],
      "theme_frame": "empty.png"
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "right top",
        "left top"
      ]
    },
    "colors": {
      "frame": "#adb09f",
      "tab_background_text": "#000"
    }
  }
}