From the blog post "Tiny Hints: a couple of Xcode 11.3 SwiftUI Preview tips"

ContentView.swift

//
//  ContentView.swift
//  PreviewsTips2020.01.24
//  Updated 2021.09.24
//
// Dad
// @GeekAndDad
//
// Code from blog post:
// https://geekanddad.wordpress.com/2020/01/24/tiny-hints-a-couple-of-xcode-11-3-swiftui-preview-tips/

import SwiftUI

struct ContentView: View {
  var body: some View {
    HStack {
      Image(systemName: "paperplane")
        .font(.largeTitle)
      VStack {
        Text("Title Text")
          .font(.title)
        Text("Subtitle goes here")
          .font(.subheadline)
      }
    }
  }
}


struct Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
      .previewLayout(.fixed(width: 400, height: 100))
  }
}


// showing the limitation of Dark Mode previews when not in a
// NavigationView

struct PreviewsDarkMode: PreviewProvider {
  static var previews: some View {
    ContentView()
      .previewLayout(.fixed(width: 400, height: 100))
      .border(Color.green)
      .preferredColorScheme(.dark)
  }
}



struct MyPreviewModifer: ViewModifier {
  var displayMode: ColorScheme = .light
  var name: String = ""
  
  func body(content: Content) -> some View {
    content
      .previewLayout(.fixed(width: 400, height: 120))
      .previewDisplayName(name)
      .preferredColorScheme(displayMode)
  }
}


struct PreviewsLightDarkDynamicText: PreviewProvider {
  static var previews: some View {
    Group {
      ContentView()
        .modifier(MyPreviewModifer())
      
      ContentView()
        .modifier(MyPreviewModifer(displayMode: .dark))
      
      ContentView()
        .modifier(MyPreviewModifer(displayMode: .dark, name: "dark extraSmall"))
        .environment(\.sizeCategory, .extraSmall)
      
      ContentView()
        .modifier(MyPreviewModifer(displayMode: .dark, name: "dark extraExtraLarge"))
        .environment(\.sizeCategory, .extraExtraLarge)
    }
  }
}

Blog post motivating this and with screenshots is here.