An example of using Xcode Preview for projects without SwiftUI.

ViewController.swift

final class ViewController: UIViewController {
}

#if canImport(SwiftUI) && DEBUG

import SwiftUI

struct ViewControllerRepresentable: UIViewRepresentable {

    func makeUIView(context: Context) -> UIView {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let viewController = storyboard.instantiateViewController(withIdentifier: "ViewController")
        return viewController.view
    }

    func updateUIView(_ view: UIView, context: Context) {

    }
}

@available(iOS 13.0, *)
struct ViewController_Preview: PreviewProvider {

    static var previews: some View {
        Group {
            ViewControllerRepresentable()
                .colorScheme(.light)
                .previewDisplayName("Light Mode")
            ViewControllerRepresentable()
                .colorScheme(.dark)
                .previewDisplayName("Dark Mode")
        }
    }
}

#endif