fix: primary action button appearance issue intro in af42540

Mastodon/Scene/Onboarding/Welcome/WelcomeViewController.swift

@@ -36,6 +36,7 @@ final class WelcomeViewController: UIViewController, NeedsDependency {
     
     private(set) lazy var  signUpButton: PrimaryActionButton = {
         let button = PrimaryActionButton()
+        button.adjustsBackgroundImageWhenUserInterfaceStyleChanges = false
         button.setTitle(L10n.Common.Controls.Actions.signUp, for: .normal)
         let backgroundImageColor: UIColor = traitCollection.userInterfaceIdiom == .phone ? .white : Asset.Colors.brandBlue.color
         let backgroundImageHighlightedColor: UIColor = traitCollection.userInterfaceIdiom == .phone ? UIColor(white: 0.8, alpha: 1.0) : Asset.Colors.brandBlueDarken20.color

Mastodon/Scene/Share/View/Button/PrimaryActionButton.swift

@@ -20,6 +20,8 @@ class PrimaryActionButton: UIButton {
     }()
     
     private var originalButtonTitle: String?
+
+    var adjustsBackgroundImageWhenUserInterfaceStyleChanges = true
     
     override init(frame: CGRect) {
         super.init(frame: frame)
@@ -51,7 +53,9 @@ extension PrimaryActionButton {
     override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
         super.traitCollectionDidChange(previousTraitCollection)
 
-        setupBackgroundAppearance()
+        if adjustsBackgroundImageWhenUserInterfaceStyleChanges {
+            setupBackgroundAppearance()
+        }
     }
     
     func showLoading() {