Example: define dynamic colors via SwiftUI's ShapeStyle on iOS 17

ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        Text("Hello, world!")
            .foregroundStyle(.customBackground)
    }
}

#Preview {
    ContentView()
}

// MARK: - Custom Colors

extension ShapeStyle where Self == CustomBackgroundColor {
    static var customBackground: CustomBackgroundColor { .init() }
}

struct CustomBackgroundColor: ShapeStyle {

    func resolve(in environment: EnvironmentValues) -> some ShapeStyle {
        if environment.colorScheme == .light {
            return Color.red
        } else {
            return Color.green
        }
    }
}