Example code showing how to set color scheme in SwiftUI app using code from https://github.com/writefreely/writefreely-swiftui-multiplatform

testColorScheme.swift

import SwiftUI

@main
struct testColorSchemeApp: App {
    @StateObject private var appearanceManager = AppearanceManager()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(appearanceManager)
                .onAppear {
                    appearanceManager.setAppearance()
                }
        }
    }
}

struct ContentView: View {
    @EnvironmentObject var appearanceManager: AppearanceManager

    var body: some View {
        NavigationView {
            List {
                Section(header: Text("Appearance")) {
                    Button {
                        appearanceManager.appearance = .system
                    } label: {
                        AppearanceOption(appearance: .system)
                    }

                    Button {
                        appearanceManager.appearance = .light
                    } label: {
                        AppearanceOption(appearance: .light)
                    }

                    Button {
                        appearanceManager.appearance = .dark
                    } label: {
                        AppearanceOption(appearance: .dark)
                    }
                }
            }
            .navigationTitle("Color Scheme")
        }
    }
}

struct AppearanceOption: View {
    @EnvironmentObject var appearanceManager: AppearanceManager
    var appearance: AppearanceManager.Appearance

    var body: some View {
        HStack {
            Text(appearance.rawValue.capitalized)
            Spacer()
            if appearanceManager.appearance == appearance {
                Image(systemName: "checkmark")
            }
        }
    }
}

class AppearanceManager: ObservableObject {
    @AppStorage("appearance") var appearance: Appearance = .system { // default appearance is .system
        didSet {
            setAppearance()
        }
    }

    enum Appearance: String {
        case light
        case dark
        case system
    }

    func setAppearance() {
        switch appearance {
        case .light:
            window?.overrideUserInterfaceStyle = .light
        case .dark:
            window?.overrideUserInterfaceStyle = .dark
        case .system:
            window?.overrideUserInterfaceStyle = .unspecified
        }
    }

    // https://github.com/writefreely/writefreely-swiftui-multiplatform/blob/main/Shared/Preferences/PreferencesModel.swift
    /* We're stuck dropping into AppKit/UIKit to set light/dark schemes for now,
     * because setting the .preferredColorScheme modifier on views in SwiftUI is
     * currently unreliable.
     *
     * Feedback submitted to Apple:
     *
     * FB8382883: "On macOS 11β4, preferredColorScheme modifier does not respect .light ColorScheme"
     * FB8383053: "On iOS 14β4/macOS 11β4, it is not possible to unset preferredColorScheme after setting
     *              it to either .light or .dark"
     */
    private var window: UIWindow? {
        guard let scene = UIApplication.shared.connectedScenes.first,
              let windowSceneDelegate = scene.delegate as? UIWindowSceneDelegate,
              let window = windowSceneDelegate.window else {
            return nil
        }
        return window
    }
}