Test for colorScheme env variable.

HelloWorldView.swift

import SwiftUI

struct ContentView: View {
	@Environment(\.colorScheme) var colorScheme: ColorScheme
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundStyle(.tint)
            Text("Hello, world!")
        }
        .padding()
		.onChange(of: colorScheme) { newValue in
			print("colorScheme [\(colorScheme)] changed to \(newValue)")
		}
		
    }
}

#Preview {
    ContentView()
}

Running and then putting into the background gets the following output:

colorScheme [light] changed to dark
colorScheme [dark] changed to light

or vice versa depending on what your current mode is.