Examine source code of Willow Theme by nekoflyy.

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "neko's theme 1",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/0.jpg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "right top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(165, 59, 82)",
      "toolbar_text": "rgb(225, 208, 20)",
      "frame": "rgb(15, 35, 159)",
      "tab_background_text": "rgb(0, 0, 0)",
      "toolbar_field": "rgba(3, 3, 3, 0.94)",
      "toolbar_field_text": "rgb(170, 170, 170)",
      "tab_line": "rgb(0, 0, 0)",
      "popup": "rgb(26, 27, 36)",
      "popup_text": "rgb(145, 114, 114)",
      "tab_loading": "rgb(0, 0, 0)"
    }
  }
}