Examine source code of Basically Nord.

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Basically Nord.",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-000-5672c42860d5b06e1058dc477397f3ef.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(46, 52, 64)",
      "toolbar_text": "rgb(216, 222, 233)",
      "frame": "rgb(59, 66, 82)",
      "tab_background_text": "rgb(236, 239, 244)",
      "toolbar_field": "rgb(67, 76, 94)",
      "toolbar_field_text": "rgb(229, 233, 240)",
      "tab_line": "rgb(94, 129, 172)",
      "popup": "rgb(46, 52, 64)",
      "popup_text": "rgb(136, 192, 208)",
      "popup_highlight": "rgb(76, 86, 106)",
      "toolbar_field_border_focus": "rgb(94, 129, 172)",
      "tab_loading": "rgb(94, 129, 172)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{7969e2e1-73c3-48d0-b330-ce6c7fa5d81c}"
    }
  }
}