Examine source code of Ice Blue-

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Ice Blue",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-000-5672c42860d5b06e1058dc477397f3ef.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(68, 64, 68)",
      "toolbar_text": "rgb(168, 190, 214)",
      "frame": "rgb(168, 190, 214)",
      "tab_background_text": "rgb(50, 57, 57)",
      "toolbar_field": "rgb(68, 64, 68)",
      "toolbar_field_text": "rgb(168, 190, 214)",
      "tab_line": "rgb(168, 190, 214)",
      "popup": "rgb(68, 64, 68)",
      "popup_text": "rgb(168, 190, 214)",
      "tab_loading": "rgb(168, 190, 214)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{e9d806b6-18ca-4730-beac-8ecc53f1f96f}"
    }
  }
}