Examine source code of Custom Dark

{
  "manifest_version": 2,
  "version": "1.2",
  "name": "Custom Dark",
  "theme": {
    "images": {},
    "properties": {},
    "colors": {
      "toolbar": "rgb(40, 40, 40)",
      "toolbar_text": "rgb(225, 225, 225)",
      "frame": "rgb(0, 0, 0)",
      "tab_background_text": "rgb(200, 200, 200)",
      "toolbar_field": "rgb(55, 55, 55)",
      "toolbar_field_text": "rgb(225, 225, 225)",
      "tab_line": "rgb(40, 40, 40)",
      "popup": "rgb(0, 0, 0)",
      "popup_text": "rgb(225, 225, 225)",
      "button_background_active": "rgb(90, 90, 90)",
      "button_background_hover": "rgb(75, 75, 75)",
      "icons_attention": "rgb(40, 225, 0)",
      "popup_border": "rgb(0, 0, 0)",
      "tab_loading": "rgb(40, 40, 40)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{93622916-9f8c-4139-b305-e6713fce3f4e}"
    }
  }
}