Examine source code of -Days Gone By-

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Days Gone By",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/0.jpg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "right top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgba(248, 237, 236, 0)",
      "toolbar_text": "rgb(249, 247, 246)",
      "frame": "rgb(243, 223, 223)",
      "tab_background_text": "rgb(4, 4, 4)",
      "toolbar_field": "rgba(255, 255, 255, 0)",
      "toolbar_field_text": "rgb(249, 247, 246)",
      "tab_line": "rgb(140, 89, 87)",
      "popup": "rgb(248, 237, 236)",
      "popup_text": "rgb(4, 0, 0)",
      "tab_loading": "rgb(140, 89, 87)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{bff25d77-c4b7-4586-b3c9-b9ccbc3406f1}"
    }
  }
}