Examine source code of juntheme-simple black

{
  "manifest_version": 2,
  "version": "2.0",
  "name": "test",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-035-ee0cb79992b920e5e4a105d0f9dca53c.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(14, 33, 47)",
      "toolbar_field": "rgb(50, 55, 55)",
      "toolbar_field_text": "rgb(240, 241, 215)",
      "tab_line": "rgb(35, 205, 179)",
      "popup": "rgb(27, 40, 45)",
      "popup_text": "rgb(140, 140, 140)",
      "tab_loading": "rgb(35, 205, 179)",
      "bookmark_text": "rgb(216, 222, 226)",
      "frame": "rgb(16, 17, 18)",
      "tab_background_text": "rgb(197, 198, 211)"
    }
  }
}