Examine source code of dark white border

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "dark white border",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-000-5672c42860d5b06e1058dc477397f3ef.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(39, 39, 50)",
      "toolbar_text": "rgb(255, 255, 255)",
      "frame": "rgb(19, 19, 25)",
      "tab_background_text": "rgb(206, 206, 199)",
      "toolbar_field": "rgba(0, 0, 0, 0.37)",
      "toolbar_field_text": "rgb(255, 255, 255)",
      "tab_line": "rgb(255, 255, 255)",
      "popup": "rgb(51, 52, 53)",
      "popup_text": "rgb(255, 255, 255)",
      "ntp_background": "rgb(39, 39, 50)",
      "ntp_text": "rgb(255, 255, 255)",
      "tab_loading": "rgb(255, 255, 255)"
    }
  }
}