Examine source code of forest_

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "forest",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/0.jpg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "right top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgba(0, 0, 0, 0.1)",
      "toolbar_text": "rgb(226, 226, 226)",
      "frame": "rgb(0, 0, 0)",
      "tab_background_text": "rgb(210, 210, 210)",
      "toolbar_field": "rgba(0, 0, 0, 0.4)",
      "toolbar_field_text": "rgb(210, 210, 210)",
      "tab_line": "rgb(255, 255, 255)",
      "popup": "rgb(12, 39, 31)",
      "popup_text": "rgb(255, 255, 255)",
      "tab_loading": "rgb(255, 255, 255)"
    }
  }
}