Examine source code of Blue Grey Thunderbird

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Blue Grey Thunderbird",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/Thunderbird.png"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "center top"
      ],
      "additional_backgrounds_tiling": [
        "repeat-x"
      ]
    },
    "colors": {
      "toolbar": "rgb(160, 168, 175)",
      "toolbar_text": "rgb(0, 0, 0)",
      "frame": "rgb(57, 59, 59)",
      "tab_background_text": "rgb(0, 0, 0)",
      "toolbar_field": "rgb(235, 235, 235)",
      "toolbar_field_text": "rgb(0, 0, 0)",
      "tab_line": "rgb(0, 148, 236)",
      "popup": "rgb(255, 255, 255)",
      "popup_text": "rgb(0, 0, 0)",
      "tab_loading": "rgb(0, 148, 236)"
    }
  }
}