[P0-①] Theme follows system + keyboard background transparent

- Split Palette.dark / Palette.light into a ThemePalette struct
- Add EnvironmentKey<ThemePalette> + ThemedRoot<Content>
- Wrap OSGKeyboardApp root in ThemedRoot so main App follows system
- Remove 6 .preferredColorScheme(.dark) overrides from main App views
- KeyboardRootView drops its Palette.background fill, uses .background(Color.clear)
- Drop the unused 'background' computed property from KeyboardRootView
- Legacy 'Palette.xxx' static accessors still resolve to dark values,
  so 2000 lines of existing call sites stay untouched

xcodebuild iOS Simulator: SUCCEEDED

OSGKeyboard/OSGKeyboardApp.swift

@@ -1,7 +1,5 @@
 // OSGKeyboardApp.swift
 // OSGKeyboard · Main App
-//
-// DEBUG VERSION 2: restore real flow but instrument every step.
 
 import SwiftUI
 import OSGKeyboardShared
@@ -11,21 +9,22 @@ struct OSGKeyboardApp: App {
     @StateObject private var config = ProviderConfig.shared
 
     init() {
+        #if DEBUG
         print("🔥 [OSGKeyboardApp] init()")
+        #endif
     }
 
     var body: some Scene {
         WindowGroup {
+            ThemedRoot {
                 Group {
                     if config.isConfigured {
                         HomeView()
-                        .onAppear { print("🔥 [OSGKeyboardApp] → HomeView appeared") }
                     } else {
                         OnboardingView(config: config)
-                        .onAppear { print("🔥 [OSGKeyboardApp] → OnboardingView appeared") }
                     }
                 }
-            .onAppear { print("🔥 [OSGKeyboardApp] body appeared") }
+            }
         }
     }
 }

OSGKeyboard/Views/HomeView.swift

@@ -33,7 +33,6 @@ struct HomeView: View {
         .sheet(isPresented: $showKeyboardPreview) {
             KeyboardPreviewSheet()
         }
-        .preferredColorScheme(.dark)
     }
 
     // MARK: - Header

OSGKeyboard/Views/KeyboardPreviewSheet.swift

@@ -42,7 +42,6 @@ struct KeyboardPreviewSheet: View {
                 keyboardBlock
             }
         }
-        .preferredColorScheme(.dark)
     }
 
     private var mockTextField: some View {

OSGKeyboard/Views/KeyboardPreviewStub.swift

@@ -30,7 +30,6 @@ struct KeyboardPreviewStub: View {
             .padding(.bottom, 6)
         }
         .frame(height: 280)
-        .preferredColorScheme(.dark)
     }
 
     // MARK: - Top bar

OSGKeyboard/Views/OnboardingView.swift

@@ -37,7 +37,6 @@ struct OnboardingView: View {
                     .padding(.bottom, Spacing.lg)
             }
         }
-        .preferredColorScheme(.dark)
     }
 
     private var pageDots: some View {

OSGKeyboard/Views/SettingsView.swift

@@ -37,7 +37,6 @@ struct SettingsView: View {
                         .foregroundStyle(Palette.accent)
                 }
             }
-            .preferredColorScheme(.dark)
         }
         .confirmationDialog(
             "Reset all settings?",

OSGKeyboardExt/Views/KeyboardRootView.swift

@@ -35,9 +35,6 @@ public struct KeyboardRootView: View {
     static let totalHeight: CGFloat = 280
 
     public var body: some View {
-        ZStack(alignment: .top) {
-            background
-
         VStack(spacing: 0) {
             topBar
                 .frame(height: 32)
@@ -50,36 +47,11 @@ public struct KeyboardRootView: View {
         }
         .padding(.top, 4)
         .padding(.bottom, 6)
-        }
+        // iOS keyboard extensions always render dark (Apple's default
+        // for custom keyboards), and we let the system UI chrome show
+        // through by drawing no background of our own.
+        .background(Color.clear)
         .frame(height: Self.totalHeight)
-        .preferredColorScheme(.dark)
-    }
-
-    // MARK: - Background
-
-    /// Solid dark fill plus a hairline highlight at the top edge, so the
-    /// keyboard reads as a physical surface rather than a floating card.
-    private var background: some View {
-        ZStack {
-            Palette.background
-            VStack(spacing: 0) {
-                Rectangle()
-                    .fill(
-                        LinearGradient(
-                            colors: [Color.white.opacity(0.05), .clear],
-                            startPoint: .top,
-                            endPoint: .bottom
-                        )
-                    )
-                    .frame(height: 1)
-                Spacer(minLength: 0)
-            }
-        }
-        .overlay(alignment: .top) {
-            Rectangle()
-                .fill(Palette.divider)
-                .frame(height: 0.5)
-        }
     }
 
     // MARK: - Top bar
@@ -182,19 +154,16 @@ extension KeyboardRootView {
 #Preview("Keyboard · Idle") {
     KeyboardRootView(state: KeyboardViewController.State.previewIdle)
         .frame(width: 390, height: 280)
-        .preferredColorScheme(.dark)
 }
 
 #Preview("Keyboard · Recording") {
     KeyboardRootView(state: KeyboardViewController.State.previewRecording)
         .frame(width: 390, height: 280)
-        .preferredColorScheme(.dark)
 }
 
 #Preview("Keyboard · Processing") {
     KeyboardRootView(state: KeyboardViewController.State.previewProcessing)
         .frame(width: 390, height: 280)
-        .preferredColorScheme(.dark)
 }
 #endif
 

OSGKeyboardShared/DesignSystem/Theme.swift

@@ -9,7 +9,37 @@
 
 import SwiftUI
 
-// MARK: - Palette
+// MARK: - Theme palette (light / dark)
+
+/// Single source of truth for *one* colour scheme. The active palette is
+/// injected via the `\.themePalette` environment key — see
+/// `DesignSystem/ThemedRoot.swift`. Token names mirror the previous
+/// `Palette` static API so existing call sites (`Palette.background` etc.)
+/// still compile and resolve through the legacy static accessors below.
+public struct ThemePalette: Sendable {
+    public let background:      Color
+    public let surface:         Color
+    public let surfaceElevated: Color
+    public let surfaceMuted:    Color
+
+    public let accent:      Color
+    public let accentMuted: Color
+    public let accentGlow:  Color
+
+    public let danger:  Color
+    public let success: Color
+    public let warning: Color
+
+    public let textPrimary:   Color
+    public let textSecondary: Color
+    public let textTertiary:  Color
+    public let textOnAccent:  Color
+
+    public let divider:       Color
+    public let dividerStrong: Color
+
+    public let recordRed: Color
+}
 
 public enum Palette {
     // Backgrounds
@@ -40,6 +70,69 @@ public enum Palette {
 
     // Recording state
     public static let recordRed       = Color(red: 1.000, green: 0.231, blue: 0.188)  // #FF3B30
+
+    /// Canonical dark palette — preserves every legacy literal above so
+    /// existing call sites that read `Palette.background` directly keep
+    /// getting the dark value (important for the keyboard extension, which
+    /// deliberately stays dark regardless of system appearance).
+    public static let dark = ThemePalette(
+        background:      background,
+        surface:         surface,
+        surfaceElevated: surfaceElevated,
+        surfaceMuted:    surfaceMuted,
+        accent:          accent,
+        accentMuted:     accentMuted,
+        accentGlow:      accentGlow,
+        danger:          danger,
+        success:         success,
+        warning:         warning,
+        textPrimary:     textPrimary,
+        textSecondary:   textSecondary,
+        textTertiary:    textTertiary,
+        textOnAccent:    textOnAccent,
+        divider:         divider,
+        dividerStrong:   dividerStrong,
+        recordRed:       recordRed
+    )
+
+    /// Light palette — iOS system light mode defaults. Used by the main app
+    /// when the user is in light mode; the keyboard extension stays dark.
+    public static let light = ThemePalette(
+        background:      Color(red: 0.980, green: 0.980, blue: 0.988),  // #FAFAFC
+        surface:         Color(red: 1.000, green: 1.000, blue: 1.000),  // #FFFFFF
+        surfaceElevated: Color(red: 0.941, green: 0.941, blue: 0.961),  // #F0F0F5
+        surfaceMuted:    Color(red: 0.953, green: 0.953, blue: 0.965),  // #F3F3F6
+        accent:          Color(red: 0.000, green: 0.478, blue: 1.000),  // iOS systemBlue
+        accentMuted:     Color(red: 0.000, green: 0.478, blue: 1.000).opacity(0.14),
+        accentGlow:      Color(red: 0.000, green: 0.478, blue: 1.000).opacity(0.32),
+        danger:          Color(red: 1.000, green: 0.231, blue: 0.188),  // #FF3B30
+        success:         Color(red: 0.157, green: 0.812, blue: 0.412),  // #28CF69
+        warning:         Color(red: 1.000, green: 0.620, blue: 0.094),  // #FF9E18
+        textPrimary:     Color(red: 0.067, green: 0.067, blue: 0.094),  // #111118
+        textSecondary:   Color(red: 0.392, green: 0.392, blue: 0.435),  // #64646F
+        textTertiary:    Color(red: 0.557, green: 0.557, blue: 0.604),  // #8E8E9A
+        textOnAccent:    Color.white,
+        divider:         Color.black.opacity(0.06),
+        dividerStrong:   Color.black.opacity(0.10),
+        recordRed:       Color(red: 1.000, green: 0.231, blue: 0.188)   // #FF3B30
+    )
+}
+
+// MARK: - Environment key
+
+private struct ThemePaletteKey: EnvironmentKey {
+    /// Default falls back to the legacy dark palette so views that haven't
+    /// been wrapped in `ThemedRoot` continue to look identical to today.
+    static let defaultValue: ThemePalette = Palette.dark
+}
+
+public extension EnvironmentValues {
+    /// The palette currently active for this view. Reads from the nearest
+    /// `ThemedRoot` ancestor (or `Palette.dark` if none).
+    var themePalette: ThemePalette {
+        get { self[ThemePaletteKey.self] }
+        set { self[ThemePaletteKey.self] = newValue }
+    }
 }
 
 // MARK: - Spacing scale (4 pt grid)

OSGKeyboardShared/DesignSystem/ThemedRoot.swift

@@ -0,0 +1,51 @@
+// ThemedRoot.swift
+// OSGKeyboard · Design System
+//
+// `ThemedRoot` injects the right `ThemePalette` for the current system
+// colour scheme. Wrap the main App's root in this view to opt into
+// light/dark following; the keyboard extension deliberately stays dark
+// (Apple's custom keyboards always render dark) and does NOT use this.
+
+import SwiftUI
+
+public struct ThemedRoot<Content: View>: View {
+
+    @Environment(\.colorScheme) private var colorScheme
+
+    let content: () -> Content
+
+    public init(@ViewBuilder content: @escaping () -> Content) {
+        self.content = content
+    }
+
+    public var body: some View {
+        content()
+            .environment(\.themePalette, colorScheme == .dark ? Palette.dark : Palette.light)
+    }
+}
+
+#if DEBUG
+#Preview("ThemedRoot · Dark") {
+    ThemedRoot {
+        ZStack {
+            Palette.dark.background.ignoresSafeArea()
+            Text("Dark")
+                .foregroundStyle(Palette.dark.textPrimary)
+                .font(.title)
+        }
+    }
+    .preferredColorScheme(.dark)
+}
+
+#Preview("ThemedRoot · Light") {
+    ThemedRoot {
+        ZStack {
+            Palette.light.background.ignoresSafeArea()
+            Text("Light")
+                .foregroundStyle(Palette.light.textPrimary)
+                .font(.title)
+        }
+    }
+    .preferredColorScheme(.light)
+}
+#endif