Chapter 2: Asset catalogs - issue with background color

I’ve found related topic in this forum, but I actually don’t get it, how to fix this problem!

To get the result like it is in book, I putted images in Assets.xcassets/Texture folder +
changed the method setupScene() to:

func setupScene() {
    ...
    scnScene.background.contents = "Textures/Background_Diffuse.png"
}