Examine source code of Grey grid

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Grey grid",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-005-f5f3cf677b1f132c75ddc7bd319dd1fc.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(223, 223, 223)",
      "toolbar_text": "rgb(255, 255, 255)",
      "frame": "rgb(182, 182, 182)",
      "tab_background_text": "rgb(239, 239, 239)",
      "toolbar_field": "rgb(237, 237, 237)",
      "toolbar_field_text": "rgb(250, 250, 250)",
      "tab_line": "rgb(252, 252, 252)",
      "popup": "rgb(189, 189, 189)",
      "popup_text": "rgb(255, 255, 255)",
      "tab_loading": "rgb(252, 252, 252)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{f77feae2-ee8e-43de-a2d2-465b93e12817}"
    }
  }
}