无法更改SwiftUI上的暗模式

import SwiftUI

struct ContentView: View {
    @Environment(\.colorScheme) var colorScheme

    var body: some View {
        VStack {
            Text("Current theme is \(colorScheme.rawValue)")
                .foregroundColor(colorScheme == .dark ? .white : .black)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .previewDevice("iPhone 12")
            .previewDisplayName("Light Mode")
            .environment(\.colorScheme, .light)

        ContentView()
            .previewDevice("iPhone 12")
            .previewDisplayName("Dark Mode")
            .environment(\.colorScheme, .dark)
    }
}

import SwiftUI

class ThemeManager: ObservableObject {
    @Published var theme: Theme = .light

    enum Theme {
        case light
        case dark
    }

    init() {
        if #available(iOS 13.0, *) {
            self.theme = UIColor.currentSystemTheme == .dark ? .dark : .light
        }
    }
}

struct ContentView: View {
    @EnvironmentObject var themeManager: ThemeManager

    var body: some View {
        VStack {
            Text("Current theme is \(themeManager.theme.rawValue)")
                .foregroundColor(themeManager.theme == .dark ? .white : .black)
        }
        .background(themeManager.theme == .dark ? Color.black : Color.white)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .previewDevice("iPhone 12")
            .previewDisplayName("Light Mode")
            .environmentObject(ThemeManager())

        ContentView()
            .previewDevice("iPhone 12")
            .previewDisplayName("Dark Mode")
            .environment(\.colorScheme, .dark)
            .environmentObject(ThemeManager())
    }
}