Examine source code of Theme for Nightly

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Theme for Nightly",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/0.jpg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "center top"
      ],
      "additional_backgrounds_tiling": [
        "no-repeat"
      ]
    },
    "colors": {
      "toolbar": "rgba(108, 0, 170, 0.7)",
      "toolbar_text": "rgb(0, 178, 178)",
      "frame": "rgb(22, 75, 106)",
      "tab_background_text": "rgb(255, 255, 255)",
      "toolbar_field": "rgba(108, 0, 170, 0)",
      "toolbar_field_text": "rgb(255, 255, 255)",
      "tab_line": "rgb(0, 178, 178)",
      "popup": "rgb(25, 1, 41)",
      "popup_text": "rgb(0, 178, 178)",
      "tab_loading": "rgb(0, 178, 178)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{2e9a35d3-dc56-4604-b9b6-fd36e71b9389}"
    }
  }
}