flutter/ios/Runner/Assets.xcassets/LaunchBackground.imageset/Contents.json

{
  "images" : [
    {
      "filename" : "background.png",
      "idiom" : "universal"
    },
    {
      "appearances" : [
        {
          "appearance" : "luminosity",
          "value" : "dark"
        }
      ],
      "filename" : "darkbackground.png",
      "idiom" : "universal"
    }
  ],
  "info" : {
    "author" : "xcode",
    "version" : 1
  }
}