[Swift][Breaking]: Make the checkout appearance own its native colors

[kieran-osgood-shopify]

What changes are you making?

Part of https://github.com/shop/issues-checkout-kit/issues/1260

Checkout colors in the Swift SDK now belong to the appearance, matching the Android SDK's CheckoutAppearance / ColorScheme / Colors model. Before, colors were separate global settings (tintColor, backgroundColor, closeButtonTintColor). Changing the appearance kept whatever colors were set before it, so a checkout could show the background of an earlier scheme.

• CheckoutAppearance (.app(ColorScheme) / .storefront(colors:)) replaces Configuration.Appearance.
• ColorScheme (.light(colors:) / .dark(colors:) / .automatic(lightColors:darkColors:)) replaces Configuration.ColorScheme.
• Colors holds webViewBackground, headerBackground, headerFont, progressIndicator, closeIcon, closeIconTint and headerBorderColor, the same fields Android has (Android also has dragHandleColor).
• Assigning an appearance replaces its colors, so .app(.automatic()) after .app(.dark(...)) uses the automatic defaults.
• On automatic, the header, title, close button, progress bar and WebView background update when the system appearance changes while checkout is open. storefront keeps the checkout light.
• The ShopifyCheckout SwiftUI .appearance(...) modifier stays instance-scoped, and a reused preloaded WebView takes the colors of the checkout presenting it.

Breaking: Configuration.tintColor, backgroundColor and closeButtonTintColor, plus the SwiftUI .tintColor, .backgroundColor and .closeButtonTintColor modifiers, are removed. platforms/swift/README.md has the migration notes.

The React Native bridge that uses this API is #597.

How to test

Run the Swift sample app (CheckoutKitSwiftDemo) on a simulator, add an item to the cart, and use Settings → Theme to pick the appearance. Toggle the system appearance with ⌘⇧A.

GIVEN the appearance is set to App dark

AND the system appearance is light

WHEN the reviewer opens checkout from the cart

THEN the header, title, close button and WebView background should all be dark

---

GIVEN the appearance was App dark and is then changed to App automatic

AND the system appearance is light

WHEN the reviewer opens checkout from the cart

THEN checkout should be fully light, with no dark background left over from App dark

---

GIVEN the appearance is set to App automatic

AND checkout is open with the system appearance light

WHEN the reviewer switches the system appearance to dark

THEN the header, title, close button and WebView background should switch to dark without reopening checkout

---

GIVEN the appearance is set to Storefront

AND the system appearance is dark

WHEN the reviewer opens checkout from the cart

THEN checkout should stay light and show the storefront's branding

---

Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

[github-actions]

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[bitrise]

Bitrise builds

E2E · iOS CI

Install this build

Open Tophat, select your target device, then click Install. Links open on the Mac running Tophat.

SDK	Install
Swift	Install with Tophat

Checkout Kit E2E results

Status	Tags	Target	Platform	OS version tag	Device
✅	launch, checkout-presentation, checkout-completion, buyer-identity, preload	swift	ios	latest	iPhone 15 iOS 27 Beta