diff --git a/GhosttyTabs.xcodeproj/project.pbxproj b/GhosttyTabs.xcodeproj/project.pbxproj
index 9300b040dcb0..2a4e643834df 100644
--- a/GhosttyTabs.xcodeproj/project.pbxproj
+++ b/GhosttyTabs.xcodeproj/project.pbxproj
@@ -135,6 +135,8 @@
 					D1000010A1B2C3D4E5F60800 /* ThemeTokens.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1000011A1B2C3D4E5F60800 /* ThemeTokens.swift */; };
 					D1000020A1B2C3D4E5F60800 /* ThemeStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1000021A1B2C3D4E5F60800 /* ThemeStore.swift */; };
 					D1000030A1B2C3D4E5F60800 /* ThemeStoreTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1000031A1B2C3D4E5F60800 /* ThemeStoreTests.swift */; };
+					D1000040A1B2C3D4E5F60800 /* AppearanceSection.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1000041A1B2C3D4E5F60800 /* AppearanceSection.swift */; };
+					D1000050A1B2C3D4E5F60800 /* BuiltInThemes.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1000051A1B2C3D4E5F60800 /* BuiltInThemes.swift */; };
 		/* End PBXBuildFile section */
 
 /* Begin PBXCopyFilesBuildPhase section */
@@ -315,6 +317,8 @@
 				D1000011A1B2C3D4E5F60800 /* ThemeTokens.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ThemeTokens.swift; sourceTree = "<group>"; };
 				D1000021A1B2C3D4E5F60800 /* ThemeStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ThemeStore.swift; sourceTree = "<group>"; };
 				D1000031A1B2C3D4E5F60800 /* ThemeStoreTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ThemeStoreTests.swift; sourceTree = "<group>"; };
+				D1000041A1B2C3D4E5F60800 /* AppearanceSection.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppearanceSection.swift; sourceTree = "<group>"; };
+				D1000051A1B2C3D4E5F60800 /* BuiltInThemes.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BuiltInThemes.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -488,6 +492,8 @@
 				D1000001A1B2C3D4E5F60800 /* SuperghostTheme.swift */,
 				D1000011A1B2C3D4E5F60800 /* ThemeTokens.swift */,
 				D1000021A1B2C3D4E5F60800 /* ThemeStore.swift */,
+				D1000041A1B2C3D4E5F60800 /* AppearanceSection.swift */,
+				D1000051A1B2C3D4E5F60800 /* BuiltInThemes.swift */,
 			);
 			path = Sources;
 			sourceTree = "<group>";
@@ -796,6 +802,8 @@
 				D1000000A1B2C3D4E5F60800 /* SuperghostTheme.swift in Sources */,
 				D1000010A1B2C3D4E5F60800 /* ThemeTokens.swift in Sources */,
 				D1000020A1B2C3D4E5F60800 /* ThemeStore.swift in Sources */,
+				D1000040A1B2C3D4E5F60800 /* AppearanceSection.swift in Sources */,
+				D1000050A1B2C3D4E5F60800 /* BuiltInThemes.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
diff --git a/Sources/AppearanceSection.swift b/Sources/AppearanceSection.swift
new file mode 100644
index 000000000000..c4cfa13f91a8
--- /dev/null
+++ b/Sources/AppearanceSection.swift
@@ -0,0 +1,225 @@
+import SwiftUI
+import AppKit
+
+// `AppearanceSection` is the new Settings → Appearance UI surface. Milestone 2 ships the
+// walking skeleton — header + live preview + preset strip with two themes. Milestones 3, 6,
+// and 7 fill in per-mode overrides, global controls, color picker, and library browser.
+//
+// Per plan §2.4 this is a section that lives inside the existing `SettingsView` ScrollView,
+// using shared `SettingsCard` / `SettingsCardRow` / `SettingsSectionHeader` components.
+// We define this in its own file because at ~600 LOC it doesn't belong inside cmuxApp.swift,
+// even though the convention for other sections is inline.
+//
+// The section reads from `ThemeStore.shared` via `@EnvironmentObject`, which is injected at
+// the `SettingsRootView` boundary. AppKit chrome (sidebar background, etc.) responds because
+// `ThemeStore.applyTheme(_:)` writes through to the legacy `sidebarTintHex*` UserDefaults
+// keys that `SidebarBackdrop` already observes; the proper migration of those readers to
+// direct `theme.tokens.*` reads is Milestone 5.
+struct AppearanceSection: View {
+    @EnvironmentObject private var themeStore: ThemeStore
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 14) {
+            SettingsAppearanceSectionHeader()
+
+            // Live preview — a miniature Superghost layout. Reacts to ThemeStore changes
+            // through @EnvironmentObject. The preview chrome (mini-sidebar, mini-terminal)
+            // pulls from `themeStore.activeTheme.tokens`; the proof that the architecture
+            // works end-to-end is that this AND the real app sidebar both recolor in the
+            // same render frame when a preset is clicked.
+            AppearanceLivePreview(theme: themeStore.activeTheme)
+                .padding(.top, 4)
+                .padding(.bottom, 8)
+
+            // Preset strip — two themes for Milestone 2 (handoff §2's curated set in
+            // M4 grows this to 8–12). Click swaps `themeStore.activeTheme` and propagates
+            // through the cache + UserDefaults bridge.
+            AppearancePresetStrip(
+                presets: BuiltInThemes.all,
+                activeId: themeStore.activeTheme.id,
+                onSelect: { theme in
+                    themeStore.applyTheme(theme)
+                }
+            )
+        }
+        .id(SettingsNavigationTarget.appearance)  // navigation target — routed via `proxy.scrollTo` in SettingsView
+    }
+}
+
+// Stable identifier (kept around in case a future caller needs to reference the section
+// outside the SettingsNavigationTarget enum).
+enum AppearanceSectionAnchor {
+    static let id = "settings.section.appearance"
+}
+
+private struct SettingsAppearanceSectionHeader: View {
+    var body: some View {
+        Text(String(localized: "settings.section.appearance", defaultValue: "Appearance"))
+            .font(.system(size: 13, weight: .semibold))
+            .foregroundColor(.secondary)
+            .padding(.leading, 2)
+            .padding(.bottom, -2)
+    }
+}
+
+// MARK: - Live preview
+
+private struct AppearanceLivePreview: View {
+    let theme: SuperghostTheme
+
+    var body: some View {
+        // The preview is a stack of two stripes — sidebar on the left, terminal pane on the
+        // right — sized to mirror the actual Superghost workspace at a glance. Picking a
+        // preset recolors both stripes in one render frame; that's the M2 acceptance demo.
+        HStack(spacing: 0) {
+            // Mini-sidebar
+            VStack(alignment: .leading, spacing: 6) {
+                miniSidebarRow(label: "main", selected: true)
+                miniSidebarRow(label: "feature/themes", selected: false)
+                miniSidebarRow(label: "test/coverage", selected: false)
+                Spacer(minLength: 0)
+            }
+            .padding(.horizontal, 10)
+            .padding(.vertical, 12)
+            .frame(width: 140, alignment: .topLeading)
+            .background(Color(nsColor: theme.tokens.cardSurface))
+
+            // Mini-terminal
+            VStack(alignment: .leading, spacing: 4) {
+                Text("$ superghost --version")
+                    .font(.system(size: 11, design: .monospaced))
+                    .foregroundColor(Color(nsColor: theme.tokens.inkBody))
+                Text("0.1.0")
+                    .font(.system(size: 11, design: .monospaced))
+                    .foregroundColor(Color(nsColor: theme.tokens.inkMuted))
+                Text("$ _")
+                    .font(.system(size: 11, design: .monospaced))
+                    .foregroundColor(Color(nsColor: theme.cursorColor))
+                Spacer(minLength: 0)
+            }
+            .padding(.horizontal, 12)
+            .padding(.vertical, 12)
+            .frame(maxWidth: .infinity, alignment: .topLeading)
+            .background(Color(nsColor: theme.backgroundColor))
+        }
+        .frame(height: 140)
+        .clipShape(RoundedRectangle(cornerRadius: 8, style: .continuous))
+        .overlay(
+            RoundedRectangle(cornerRadius: 8, style: .continuous)
+                .stroke(Color(nsColor: theme.tokens.hairlineBorder), lineWidth: 1)
+        )
+        .accessibilityElement(children: .ignore)
+        .accessibilityLabel(
+            String(
+                localized: "settings.appearance.preview.label",
+                defaultValue: "Live preview of the \(theme.name) theme"
+            )
+        )
+    }
+
+    @ViewBuilder
+    private func miniSidebarRow(label: String, selected: Bool) -> some View {
+        HStack(spacing: 6) {
+            Circle()
+                .fill(Color(nsColor: selected ? theme.tokens.accentInline : theme.tokens.inkMuted))
+                .frame(width: 6, height: 6)
+            Text(label)
+                .font(.system(size: 11, weight: selected ? .semibold : .regular))
+                .foregroundColor(Color(nsColor: selected ? theme.tokens.inkHeadline : theme.tokens.inkBody))
+                .lineLimit(1)
+            Spacer(minLength: 0)
+        }
+        .padding(.horizontal, 8)
+        .padding(.vertical, 4)
+        .background(
+            RoundedRectangle(cornerRadius: 5, style: .continuous)
+                .fill(Color(nsColor: selected ? theme.tokens.liftedSurface : .clear))
+        )
+    }
+}
+
+// MARK: - Preset strip
+
+private struct AppearancePresetStrip: View {
+    let presets: [SuperghostTheme]
+    let activeId: String
+    let onSelect: (SuperghostTheme) -> Void
+
+    var body: some View {
+        ScrollView(.horizontal, showsIndicators: false) {
+            HStack(spacing: 10) {
+                ForEach(presets, id: \.id) { theme in
+                    AppearancePresetCard(
+                        theme: theme,
+                        isActive: theme.id == activeId,
+                        onSelect: { onSelect(theme) }
+                    )
+                }
+            }
+            .padding(.vertical, 4)
+        }
+    }
+}
+
+private struct AppearancePresetCard: View {
+    let theme: SuperghostTheme
+    let isActive: Bool
+    let onSelect: () -> Void
+
+    var body: some View {
+        Button(action: onSelect) {
+            VStack(alignment: .leading, spacing: 8) {
+                // Color preview row — chrome surface + accent + ink, the tokens that most
+                // visibly distinguish themes at a glance.
+                HStack(spacing: 4) {
+                    swatch(theme.tokens.cardSurface)
+                    swatch(theme.backgroundColor)
+                    swatch(theme.tokens.accentInline)
+                    swatch(theme.tokens.inkBody)
+                }
+                Text(theme.name)
+                    .font(.system(size: 12, weight: .medium))
+                    .foregroundColor(.primary)
+                    .lineLimit(1)
+                Text(modeLabel(theme.mode))
+                    .font(.system(size: 11))
+                    .foregroundColor(.secondary)
+            }
+            .padding(10)
+            .frame(width: 144, alignment: .leading)
+            .background(
+                RoundedRectangle(cornerRadius: 8, style: .continuous)
+                    .fill(Color(nsColor: NSColor.controlBackgroundColor).opacity(0.5))
+            )
+            .overlay(
+                RoundedRectangle(cornerRadius: 8, style: .continuous)
+                    .stroke(
+                        isActive ? Color.accentColor : Color(nsColor: NSColor.separatorColor).opacity(0.5),
+                        lineWidth: isActive ? 2 : 1
+                    )
+            )
+        }
+        .buttonStyle(.plain)
+        .accessibilityLabel(theme.name)
+        .accessibilityAddTraits(isActive ? [.isSelected] : [])
+    }
+
+    private func swatch(_ color: NSColor) -> some View {
+        RoundedRectangle(cornerRadius: 3, style: .continuous)
+            .fill(Color(nsColor: color))
+            .frame(width: 22, height: 22)
+            .overlay(
+                RoundedRectangle(cornerRadius: 3, style: .continuous)
+                    .stroke(Color.black.opacity(0.08), lineWidth: 0.5)
+            )
+    }
+
+    private func modeLabel(_ mode: ThemeMode) -> String {
+        switch mode {
+        case .light:
+            return String(localized: "settings.appearance.preset.mode.light", defaultValue: "Light mode")
+        case .dark:
+            return String(localized: "settings.appearance.preset.mode.dark", defaultValue: "Dark mode")
+        }
+    }
+}
diff --git a/Sources/BuiltInThemes.swift b/Sources/BuiltInThemes.swift
new file mode 100644
index 000000000000..638907ba4ef5
--- /dev/null
+++ b/Sources/BuiltInThemes.swift
@@ -0,0 +1,135 @@
+import Foundation
+import AppKit
+
+// Curated built-in themes shipped with Superghost. The Milestone 2 walking skeleton needs
+// at least two themes (one dark, one light) to prove the architecture; Milestone 4 grows
+// this to the full 8–12 set per handoff §2 and validates contrast on every chrome token.
+//
+// Color values for each theme come from:
+//   - Tokyo Night: handoff §2 "Token Architecture", values are the published Tokyo Night
+//     palette (https://github.com/folke/tokyonight.nvim) with chrome derived for the
+//     specific ladder Matt's prototype calls out.
+//   - Catppuccin Latte: published Catppuccin Latte palette
+//     (https://github.com/catppuccin/catppuccin#-palette) with chrome chosen to mirror the
+//     Tokyo Night ladder in the inverse direction.
+//
+// Both are *fully specified* (not derived). That's the contract M4 codifies for every
+// built-in: ship every chrome token with a hand-chosen value so the curated set is the
+// reference for what "good" looks like.
+enum BuiltInThemes {
+    static var all: [SuperghostTheme] {
+        [tokyoNight, catppuccinLatte]
+    }
+
+    static let tokyoNight = SuperghostTheme(
+        id: "tokyo-night",
+        name: "Tokyo Night",
+        mode: .dark,
+        source: .builtIn,
+        backgroundColor: NSColor(hex: "#1a1b26")!,
+        foregroundColor: NSColor(hex: "#c0caf5")!,
+        cursorColor: NSColor(hex: "#7aa2f7")!,
+        cursorTextColor: NSColor(hex: "#1a1b26")!,
+        selectionBackground: NSColor(hex: "#283457")!,
+        selectionForeground: NSColor(hex: "#c0caf5")!,
+        palette: [
+            0: NSColor(hex: "#15161e")!,
+            1: NSColor(hex: "#f7768e")!,
+            2: NSColor(hex: "#9ece6a")!,
+            3: NSColor(hex: "#e0af68")!,
+            4: NSColor(hex: "#7aa2f7")!,
+            5: NSColor(hex: "#bb9af7")!,
+            6: NSColor(hex: "#7dcfff")!,
+            7: NSColor(hex: "#a9b1d6")!,
+            8: NSColor(hex: "#414868")!,
+            9: NSColor(hex: "#f7768e")!,
+            10: NSColor(hex: "#9ece6a")!,
+            11: NSColor(hex: "#e0af68")!,
+            12: NSColor(hex: "#7aa2f7")!,
+            13: NSColor(hex: "#bb9af7")!,
+            14: NSColor(hex: "#7dcfff")!,
+            15: NSColor(hex: "#c0caf5")!
+        ],
+        cardSurface: NSColor(hex: "#1a1b26")!,
+        liftedSurface: NSColor(hex: "#1f2335")!,
+        hairlineBorder: NSColor(hex: "#2f334d")!,
+        hairlineBorderHover: NSColor(hex: "#3d4263")!,
+        inkHeadline: NSColor(hex: "#c0caf5")!,
+        inkBody: NSColor(hex: "#a9b1d6")!,
+        // Bumped from the handoff's #565f89 (R1 in the plan — fails AA on caption sizes).
+        // The plan defers this to M6; we ship the safe value from the start so the walking
+        // skeleton already meets AA. Documented in the plan-fix log.
+        inkMuted: NSColor(hex: "#737aa2")!,
+        inkCaption: NSColor(hex: "#737aa2")!,
+        accentSolid: NSColor(hex: "#3d59a1")!,
+        accentInline: NSColor(hex: "#7aa2f7")!,
+        semanticSuccess: NSColor(hex: "#1a2b25")!,
+        semanticWarning: NSColor(hex: "#3d3328")!,
+        semanticDanger: NSColor(hex: "#3c2c3a")!,
+        semanticInfo: NSColor(hex: "#16263a")!,
+        semanticSkill: NSColor(hex: "#3d4263")!,
+        translucentSidebar: true,
+        contrastBoost: 68
+    )
+
+    static let catppuccinLatte = SuperghostTheme(
+        id: "catppuccin-latte",
+        name: "Catppuccin Latte",
+        mode: .light,
+        source: .builtIn,
+        backgroundColor: NSColor(hex: "#eff1f5")!,
+        foregroundColor: NSColor(hex: "#4c4f69")!,
+        cursorColor: NSColor(hex: "#1e66f5")!,
+        cursorTextColor: NSColor(hex: "#eff1f5")!,
+        selectionBackground: NSColor(hex: "#bcc0cc")!,
+        selectionForeground: NSColor(hex: "#4c4f69")!,
+        palette: [
+            0: NSColor(hex: "#5c5f77")!,
+            1: NSColor(hex: "#d20f39")!,
+            2: NSColor(hex: "#40a02b")!,
+            3: NSColor(hex: "#df8e1d")!,
+            4: NSColor(hex: "#1e66f5")!,
+            5: NSColor(hex: "#ea76cb")!,
+            6: NSColor(hex: "#179299")!,
+            7: NSColor(hex: "#acb0be")!,
+            8: NSColor(hex: "#6c6f85")!,
+            9: NSColor(hex: "#d20f39")!,
+            10: NSColor(hex: "#40a02b")!,
+            11: NSColor(hex: "#df8e1d")!,
+            12: NSColor(hex: "#1e66f5")!,
+            13: NSColor(hex: "#ea76cb")!,
+            14: NSColor(hex: "#179299")!,
+            15: NSColor(hex: "#bcc0cc")!
+        ],
+        cardSurface: NSColor(hex: "#e6e9ef")!,
+        liftedSurface: NSColor(hex: "#dce0e8")!,
+        hairlineBorder: NSColor(hex: "#ccd0da")!,
+        hairlineBorderHover: NSColor(hex: "#bcc0cc")!,
+        inkHeadline: NSColor(hex: "#4c4f69")!,
+        inkBody: NSColor(hex: "#5c5f77")!,
+        inkMuted: NSColor(hex: "#6c6f85")!,
+        inkCaption: NSColor(hex: "#7c7f93")!,
+        accentSolid: NSColor(hex: "#1e66f5")!,
+        accentInline: NSColor(hex: "#1e66f5")!,
+        semanticSuccess: NSColor(hex: "#d9eedb")!,
+        semanticWarning: NSColor(hex: "#f5e6c8")!,
+        semanticDanger: NSColor(hex: "#f5d6dc")!,
+        semanticInfo: NSColor(hex: "#d7e3fc")!,
+        semanticSkill: NSColor(hex: "#e0e3ef")!,
+        translucentSidebar: true,
+        contrastBoost: 60
+    )
+
+    // Resolve a preset by id. Used by `ThemeStore` and `AppearanceSection`.
+    static func preset(withId id: String) -> SuperghostTheme? {
+        all.first(where: { $0.id == id })
+    }
+
+    // Default theme to apply for a given color scheme when the user hasn't picked one yet.
+    static func defaultPreset(for mode: ThemeMode) -> SuperghostTheme {
+        switch mode {
+        case .dark: return tokyoNight
+        case .light: return catppuccinLatte
+        }
+    }
+}
diff --git a/Sources/ThemeStore.swift b/Sources/ThemeStore.swift
index cadb13d7a82d..744a0ffdf61f 100644
--- a/Sources/ThemeStore.swift
+++ b/Sources/ThemeStore.swift
@@ -61,6 +61,7 @@ final class ThemeStore: ObservableObject {
     // changing the call sites.
     func applyTheme(_ theme: SuperghostTheme) {
         applyToGhosttyConfigCache(theme: theme, scheme: resolvedColorScheme)
+        applyToLegacySidebarDefaults(theme: theme)
         self.activeTheme = theme
         NotificationCenter.default.post(name: Self.changeNotification, object: self)
         scheduleFileSync()
@@ -132,6 +133,29 @@ final class ThemeStore: ObservableObject {
         // Intentionally empty — see `Sources/AppearanceFileSync.swift` (M3).
     }
 
+    // M2 bridge: write the active theme's sidebar background through the legacy
+    // `sidebarTintHex*` UserDefaults keys. `SidebarBackdrop` (in `ContentView.swift`)
+    // already reads these via `@AppStorage`, so writing here is enough to make the actual
+    // app sidebar respond to a preset switch — which is the M2 acceptance gate.
+    //
+    // The proper migration of `SidebarBackdrop` to read from `themeStore.activeTheme.tokens`
+    // directly is Milestone 5 (chrome migration). Until then, the legacy UserDefaults keys
+    // remain the source of truth for those readers. The plan's §2.5 compatibility contract
+    // is honored — keys stay frozen, and we write the values the reader expects.
+    private func applyToLegacySidebarDefaults(theme: SuperghostTheme) {
+        let defaults = UserDefaults.standard
+        let hex = theme.cardSurface.hexString()
+        defaults.set(hex, forKey: "sidebarTintHex")
+        // Per-mode keys are set only for the theme's mode; the other mode is left alone so
+        // a dark-mode pick doesn't clobber the user's light-mode preference (and vice-versa).
+        switch theme.mode {
+        case .dark:
+            defaults.set(hex, forKey: "sidebarTintHexDark")
+        case .light:
+            defaults.set(hex, forKey: "sidebarTintHexLight")
+        }
+    }
+
     // MARK: - Preference persistence
 
     // `schemePreference` is stored under a stable UserDefaults key. We deliberately don't
diff --git a/Sources/cmuxApp.swift b/Sources/cmuxApp.swift
index a9e7c5ce6758..610242064cc6 100644
--- a/Sources/cmuxApp.swift
+++ b/Sources/cmuxApp.swift
@@ -2750,6 +2750,7 @@ enum SettingsNavigationTarget: String {
     case browser
     case browserImport
     case keyboardShortcuts
+    case appearance
 }
 
 enum SettingsNavigationRequest {
@@ -4939,6 +4940,8 @@ struct SettingsView: View {
                         .disabled(sidebarHideAllDetails)
                     }
 
+                    AppearanceSection()
+
                     SettingsSectionHeader(title: String(localized: "settings.section.workspaceColors", defaultValue: "Workspace Colors"))
                     SettingsCard {
                         SettingsPickerRow(
@@ -6448,6 +6451,7 @@ private struct ShortcutSettingRow: View {
 private struct SettingsRootView: View {
     var body: some View {
         SettingsView()
+            .environmentObject(ThemeStore.shared)
             .background(WindowAccessor { window in
                 configureSettingsWindow(window)
             })
diff --git a/cmuxTests/ThemeStoreTests.swift b/cmuxTests/ThemeStoreTests.swift
index 178e82d87173..295a76cf8156 100644
--- a/cmuxTests/ThemeStoreTests.swift
+++ b/cmuxTests/ThemeStoreTests.swift
@@ -187,4 +187,72 @@ final class ThemeStoreFoundationTests: XCTestCase {
             "accentInline should be drawn from the palette's accent swatches"
         )
     }
+
+    // MARK: - Milestone 2: Preset → cache + UserDefaults bridge
+
+    @MainActor
+    func testApplyTokyoNightPresetWritesCardSurfaceToLegacySidebarDefaults() {
+        // M2 acceptance: clicking a preset must propagate the new theme's chrome to the
+        // legacy sidebar UserDefaults keys so existing readers (SidebarBackdrop in
+        // ContentView.swift) update without a restart. This is the bridge the walking
+        // skeleton relies on until M5 swaps SidebarBackdrop to read from ThemeStore directly.
+        let defaults = UserDefaults.standard
+        let priorTint = defaults.string(forKey: "sidebarTintHex")
+        let priorDark = defaults.string(forKey: "sidebarTintHexDark")
+        defer {
+            // Restore so unrelated tests aren't poisoned.
+            if let priorTint { defaults.set(priorTint, forKey: "sidebarTintHex") }
+            else { defaults.removeObject(forKey: "sidebarTintHex") }
+            if let priorDark { defaults.set(priorDark, forKey: "sidebarTintHexDark") }
+            else { defaults.removeObject(forKey: "sidebarTintHexDark") }
+        }
+
+        ThemeStore.shared.applyTheme(BuiltInThemes.tokyoNight)
+
+        let expected = BuiltInThemes.tokyoNight.cardSurface.hexString()
+        XCTAssertEqual(
+            defaults.string(forKey: "sidebarTintHex"),
+            expected,
+            "applyTheme should write the active theme's cardSurface to sidebarTintHex"
+        )
+        XCTAssertEqual(
+            defaults.string(forKey: "sidebarTintHexDark"),
+            expected,
+            "applyTheme(dark) should also write to the per-mode dark key"
+        )
+    }
+
+    @MainActor
+    func testApplyLightPresetDoesNotClobberDarkSidebarDefault() {
+        // If the user has a dark sidebar pinned and we apply a light theme, the per-mode
+        // dark key must be untouched. The plan's §2.5 compatibility contract requires this.
+        let defaults = UserDefaults.standard
+        let priorDark = defaults.string(forKey: "sidebarTintHexDark")
+        defaults.set("#1a1b26", forKey: "sidebarTintHexDark")
+        defer {
+            if let priorDark { defaults.set(priorDark, forKey: "sidebarTintHexDark") }
+            else { defaults.removeObject(forKey: "sidebarTintHexDark") }
+        }
+
+        ThemeStore.shared.applyTheme(BuiltInThemes.catppuccinLatte)
+        XCTAssertEqual(
+            defaults.string(forKey: "sidebarTintHexDark"),
+            "#1a1b26",
+            "applying a light theme should not modify the per-mode dark sidebar key"
+        )
+    }
+
+    func testBuiltInThemesAreFullySpecifiedAndAACompliant() {
+        // Every curated theme must hand-specify every chrome token (handoff §2 contract for
+        // built-ins) and meet AA contrast for body ink on card surface. M4 expands this set;
+        // the contract holds for every addition.
+        for theme in BuiltInThemes.all {
+            let ratio = ThemeTokens.contrastRatio(theme.inkBody, theme.cardSurface)
+            XCTAssertGreaterThanOrEqual(
+                ratio,
+                4.5,
+                "Theme \(theme.id) must clear AA contrast for inkBody on cardSurface (was \(ratio))"
+            )
+        }
+    }
 }