Enforce theme at the window level instead of preferredColorScheme

SwiftUI's .preferredColorScheme on the root view could not revert from
dark back to light/system while a sheet was presented, and on macOS it
never applied to secondary window scenes such as Settings. The override
is now applied directly to UIKit/AppKit instead.

- SettingsManager.applyTheme sets overrideUserInterfaceStyle on every
  window of every connected scene (iOS) or NSApp.appearance (macOS);
  the theme setter calls it, so any future code path that changes the
  theme outside the setter must do the same
- AppTheme gains userInterfaceStyle / appearance mappings, with .system
  resolving to .unspecified / nil so the OS setting takes over again
- Applied on app appear for the initial launch state, and after both
  iCloud sync paths, since theme can arrive as a synced change and is
  no longer driven by SwiftUI state

Claude-Session: https://claude.ai/code/session_0154KH8RAVAvm6iVanhmoW8o

Yattee/Core/Settings/SettingsManager+CloudSync.swift

@@ -389,6 +389,10 @@ extension SettingsManager {
 
         clearCache()
         updateLastSyncTime()
+
+        #if !os(tvOS)
+        Self.applyTheme(theme)
+        #endif
     }
 
     /// Refreshes settings from iCloud by copying iCloud values to local storage.
@@ -445,5 +449,11 @@ extension SettingsManager {
 
         // Clear caches to force re-read from local storage
         clearCache()
+
+        // Re-apply the theme in case it was among the synced changes —
+        // it is enforced at the window level, not via SwiftUI state.
+        #if !os(tvOS)
+        Self.applyTheme(theme)
+        #endif
     }
 }

Yattee/Core/Settings/SettingsManager+General.swift

@@ -24,9 +24,28 @@ extension SettingsManager {
         set {
             _theme = newValue
             set(newValue.rawValue, for: .theme)
+            Self.applyTheme(newValue)
         }
     }
 
+    /// Forces the theme onto the platform windows directly. SwiftUI's
+    /// `.preferredColorScheme` fails to revert from dark back to light/system
+    /// while a sheet is presented, and on macOS it never covered secondary
+    /// window scenes (Settings), so the override is applied at the
+    /// UIKit/AppKit level instead.
+    static func applyTheme(_ theme: AppTheme) {
+        #if os(iOS)
+        for scene in UIApplication.shared.connectedScenes {
+            guard let windowScene = scene as? UIWindowScene else { continue }
+            for window in windowScene.windows {
+                window.overrideUserInterfaceStyle = theme.userInterfaceStyle
+            }
+        }
+        #elseif os(macOS)
+        NSApp.appearance = theme.appearance
+        #endif
+    }
+
     var accentColor: AccentColor {
         get {
             if let cached = _accentColor { return cached }

Yattee/Core/Settings/SettingsTypes.swift

@@ -7,6 +7,11 @@
 
 import Foundation
 import SwiftUI
+#if canImport(UIKit)
+import UIKit
+#elseif canImport(AppKit)
+import AppKit
+#endif
 
 // MARK: - Theme & Appearance
 
@@ -22,6 +27,24 @@ enum AppTheme: String, CaseIterable, Codable {
         case .dark: return .dark
         }
     }
+
+    #if canImport(UIKit)
+    var userInterfaceStyle: UIUserInterfaceStyle {
+        switch self {
+        case .system: return .unspecified
+        case .light: return .light
+        case .dark: return .dark
+        }
+    }
+    #elseif canImport(AppKit)
+    var appearance: NSAppearance? {
+        switch self {
+        case .system: return nil
+        case .light: return NSAppearance(named: .aqua)
+        case .dark: return NSAppearance(named: .darkAqua)
+        }
+    }
+    #endif
 }
 
 enum AccentColor: String, CaseIterable, Codable {

Yattee/YatteeApp.swift

@@ -70,7 +70,6 @@ struct YatteeApp: App {
             ContentView()
                 .appEnvironment(appEnvironment)
                 #if !os(tvOS)
-                .preferredColorScheme(appEnvironment.settingsManager.theme.colorScheme)
                 .tint(appEnvironment.settingsManager.resolvedAccentColor)
                 #endif
                 #if os(macOS)
@@ -85,6 +84,9 @@ struct YatteeApp: App {
                     handleContinuedActivity(activity)
                 }
                 .onAppear {
+                    #if !os(tvOS)
+                    SettingsManager.applyTheme(appEnvironment.settingsManager.theme)
+                    #endif
                     registerBackgroundTasksIfNeeded()
                     #if os(tvOS)
                     TopShelfSnapshotWriter.startObserving(dataManager: appEnvironment.dataManager)