Dark mode in SwiftUI Preview doesn't have a dark background with Xcode 11.4

Setting \.colorScheme in the environment is deprecated, so instead use the .preferedColorScheme modifier. For example,

ContentView()
    .preferredColorScheme(.dark)

As m-reza-f mentioned in a similar question, this is a bug in Xcode (which is still active as this answer is posted).

I'll add that instead of wrapping your actual body code in a NavigationView, you can simply wrap the previews code in your PreviewProvider in a NavigationView instead, to achieve the same results:

struct ContentView_Previews: PreviewProvider {
     Group {
          NavigationView {
               ContentView()
                    .environment(\.colorScheme, .light)
          }

          NavigationView {
               ContentView()
                    .environment(\.colorScheme, .light)
          }
     }
}

try this:

@available(iOS 13.0, *)
public struct DarkView<Content> : View where Content : View {
    var darkContent: Content
    var on: Bool
    public init(_ on: Bool, @ViewBuilder content: () -> Content) {
        self.darkContent = content()
        self.on = on
    }

    public var body: some View {
        ZStack {
            if on {
                Spacer()
                    .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: .infinity)
                    .background(Color.black)
                    .edgesIgnoringSafeArea(.all)
                darkContent.frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: .infinity).background(Color.black).colorScheme(.dark)
            } else {
                darkContent
            }
        }
    }
}

@available(iOS 13.0, *)
extension View {
    @available(iOS 13.0, *)
    public func darkModeFix(_ on: Bool = true) -> DarkView<Self> {
        DarkView(on) {
            self
        }
    }
}

and then

yourView()
.environment(\.colorScheme, .dark)
                    .darkModeFix()