Fix background colors in UIKit demos to work in light and dark mode (#482)

Author: hungtruong

Examples/CaseStudies/UIKitCaseStudies/CounterViewController.swift

@@ -40,7 +40,7 @@ final class CounterViewController: UIViewController {
   override func viewDidLoad() {
     super.viewDidLoad()
 
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let decrementButton = UIButton(type: .system)
     decrementButton.addTarget(self, action: #selector(decrementButtonTapped), for: .touchUpInside)

Examples/CaseStudies/UIKitCaseStudies/Internal/ActivityIndicatorViewController.swift

@@ -4,7 +4,7 @@ final class ActivityIndicatorViewController: UIViewController {
   override func viewDidLoad() {
     super.viewDidLoad()
 
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let activityIndicator = UIActivityIndicatorView()
     activityIndicator.startAnimating()

Examples/CaseStudies/UIKitCaseStudies/LoadThenNavigate.swift

@@ -67,7 +67,7 @@ class LazyNavigationViewController: UIViewController {
 
     self.title = "Load then navigate"
 
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let button = UIButton(type: .system)
     button.addTarget(self, action: #selector(loadOptionalCounterTapped), for: .touchUpInside)

Examples/CaseStudies/UIKitCaseStudies/NavigateAndLoad.swift

@@ -67,7 +67,7 @@ class EagerNavigationViewController: UIViewController {
 
     self.title = "Navigate and load"
 
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let button = UIButton(type: .system)
     button.addTarget(self, action: #selector(loadOptionalCounterTapped), for: .touchUpInside)

Examples/TicTacToe/Sources/Views-UIKit/GameViewController.swift

@@ -28,7 +28,7 @@ public final class GameViewController: UIViewController {
     super.viewDidLoad()
 
     self.navigationItem.title = "Tic-Tac-Toe"
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     self.navigationItem.leftBarButtonItem = UIBarButtonItem(
       title: "Quit",

Examples/TicTacToe/Sources/Views-UIKit/LoginViewController.swift

@@ -41,7 +41,7 @@ class LoginViewController: UIViewController {
     super.viewDidLoad()
 
     self.navigationItem.title = "Login"
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let disclaimerLabel = UILabel()
     disclaimerLabel.text = """

Examples/TicTacToe/Sources/Views-UIKit/TwoFactorViewController.swift

@@ -34,7 +34,7 @@ public final class TwoFactorViewController: UIViewController {
   public override func viewDidLoad() {
     super.viewDidLoad()
 
-    self.view.backgroundColor = .white
+    self.view.backgroundColor = .systemBackground
 
     let titleLabel = UILabel()
     titleLabel.text = "Enter the one time code to continue"