Examine source code of mytheme_clean

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "mytheme",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-004-411c27df18d2f86bd8790aa4a1bde85f.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(59, 59, 59)",
      "toolbar_text": "rgb(231, 242, 244)",
      "frame": "rgb(31, 31, 31)",
      "tab_background_text": "rgb(231, 242, 244)",
      "toolbar_field": "rgb(31, 31, 31)",
      "toolbar_field_text": "rgb(217, 242, 247)",
      "tab_line": "rgba(217, 242, 247, 0.47)",
      "popup": "rgb(59, 59, 59)",
      "popup_text": "rgb(231, 242, 244)",
      "tab_loading": "rgba(217, 242, 247, 0.47)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{6348ab9a-927f-4f21-a3ef-78a47c65ad37}"
    }
  }
}