Custom color scheme view modifier for SwiftUI. Sets the color scheme to light mode, dark mode, or matches the system. If set to match, the app will respond to system-wide color scheme changes properly. A simple widget color scheme implementation is also provided.

CustomColorScheme.swift

import SwiftUI

enum CustomColorScheme: Int, CaseIterable, Identifiable, Codable {
    static var defaultKey = "customColorScheme"
    static var defaultValue = CustomColorScheme.system
    
    case system = 0
    case light = 1
    case dark = 2
    
    var id: Int {
        self.rawValue
    }
    
    var colorScheme: ColorScheme? {
        switch self {
        case .system:
            return nil
        case .light:
            return .light
        case .dark:
            return .dark
        }
    }
    
    var label: String {
        switch self {
        case .system:
            return "System"
        case .light:
            return "Light"
        case .dark:
            return "Dark"
        }
    }
}

CustomColorSchemeExampleView.swift

import SwiftUI

struct CustomColorSchemeExampleView: View {
    // Store custom color scheme in UserDefaults
    @AppStorage(CustomColorScheme.defaultKey) var customColorScheme = CustomColorScheme.defaultValue
    
    var body: some View {
        Text("Hello World")
            .customColorScheme($customColorScheme)
    }
}

CustomColorSchemeViewModifier.swift

import SwiftUI

struct CustomColorSchemeViewModifier: ViewModifier {
    // This variable holds the currently active colour scheme (changed with calls to preferredColorScheme)
    @Environment(\.colorScheme) var colorScheme
    
    @Binding var customColorScheme: CustomColorScheme
    
    // Temp value used for changing colorScheme when switching customColorScheme to .system
    @State private var tempColorScheme: ColorScheme? = nil
    
    init(_ customColorScheme: Binding<CustomColorScheme>) {
        self._customColorScheme = customColorScheme
    }

    // This function is required to get the system color scheme
    func getSystemColorScheme() -> ColorScheme {
        return UITraitCollection.current.userInterfaceStyle == .light ? .light : .dark
    }

    func body(content: Content) -> some View {
        content
            .preferredColorScheme(tempColorScheme ?? customColorScheme.colorScheme)
            .onChange(of: customColorScheme) { value in
                if value == .system {
                    tempColorScheme = getSystemColorScheme()
                }
            }
            .onReceive(NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)) { _ in
                if customColorScheme == .system {
                    let systemColorScheme = getSystemColorScheme()
                    if systemColorScheme != colorScheme {
                        tempColorScheme = systemColorScheme
                    }
                }
            }
            .onChange(of: tempColorScheme) { value in
                if value != nil {
                    DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
                        // Resets tempColorScheme back to nil. This occurs after colorScheme has been updated
                        tempColorScheme = nil
                    }
                }
            }
    }
}

extension View {
    func customColorScheme(_ customColorScheme: Binding<CustomColorScheme>) -> some View {
        self.modifier(CustomColorSchemeViewModifier(customColorScheme))
    }
}

WidgetCustomColorSchemeExampleView.swift

import SwiftUI
import WidgetKit

struct WidgetCustomColorSchemeExampleView: View {
    // Use an app group to store and let users change it by referencing the same variable in your app.
    @AppStorage(CustomColorScheme.defaultKey, store: "com.your.groupstore") var customColorScheme = CustomColorScheme.defaultValue
    
    var body: some View {
        Text("Hello World")
            .widgetCustomColorScheme(customColorScheme)
    }
}

WidgetCustomColorSchemeViewModifier.swift

import SwiftUI
import WidgetKit

struct WidgetCustomColorSchemeViewModifier: ViewModifier {
    // As we're not changing the preferred color scheme we can trust this to always be the system value
    @Environment(\.colorScheme) var colorScheme
    let customColorScheme: CustomColorScheme
    
    init(_ customColorScheme: CustomColorScheme) {
        self.customColorScheme = customColorScheme
    }

    func body(content: Content) -> some View {
        content
            // .preferredColorScheme doesn't work on widgets
            .colorScheme(customColorScheme.colorScheme ?? colorScheme)
    }
}

extension View {
    func widgetCustomColorScheme(_ customColorScheme: CustomColorScheme) -> some View {
        self.modifier(WidgetCustomColorSchemeViewModifier(customColorScheme))
    }
}