feat: add a system appearance that follows macOS

Dory/DesignSystem/Theme.swift

@@ -1,10 +1,77 @@
+import AppKit
+import Observation
 import SwiftUI
 
 enum DoryAppearance: String, CaseIterable, Sendable {
-    case light, dark
+    case light, dark, system
 
-    var palette: DoryPalette { self == .dark ? .dark : .light }
-    var colorScheme: ColorScheme { self == .dark ? .dark : .light }
+    var label: String {
+        switch self {
+        case .light: "Light"
+        case .dark: "Dark"
+        case .system: "System"
+        }
+    }
+
+    func resolved(systemIsDark: Bool) -> DoryAppearance {
+        self == .system ? (systemIsDark ? .dark : .light) : self
+    }
+
+    var palette: DoryPalette { self == .light ? .light : .dark }
+
+    /// `nil` for `.system` so SwiftUI keeps following the OS setting.
+    var colorScheme: ColorScheme? {
+        switch self {
+        case .light: .light
+        case .dark: .dark
+        case .system: nil
+        }
+    }
+}
+
+/// Tracks the OS light/dark setting so `DoryAppearance.system` follows it live.
+@MainActor
+@Observable
+final class DorySystemAppearance {
+    static let shared = DorySystemAppearance()
+
+    private(set) var isDark: Bool = DorySystemAppearance.currentIsDark()
+    @ObservationIgnored private var observation: NSKeyValueObservation?
+
+    private init() {
+        observe()
+        if NSApp == nil {
+            // The app object does not exist yet during early launch; attach once it does.
+            NotificationCenter.default.addObserver(
+                forName: NSApplication.didFinishLaunchingNotification,
+                object: nil,
+                queue: .main
+            ) { _ in
+                MainActor.assumeIsolated {
+                    DorySystemAppearance.shared.observe()
+                    DorySystemAppearance.shared.refresh()
+                }
+            }
+        }
+    }
+
+    private func observe() {
+        guard observation == nil, let app = NSApp else { return }
+        observation = app.observe(\.effectiveAppearance, options: [.new]) { _, _ in
+            // KVO for effectiveAppearance is delivered on the main thread.
+            MainActor.assumeIsolated { DorySystemAppearance.shared.refresh() }
+        }
+    }
+
+    private func refresh() {
+        let value = Self.currentIsDark()
+        if value != isDark { isDark = value }
+    }
+
+    private static func currentIsDark() -> Bool {
+        let appearance = NSApp?.effectiveAppearance ?? NSAppearance.currentDrawing()
+        return appearance.bestMatch(from: [.aqua, .darkAqua]) == .darkAqua
+    }
 }
 
 struct DoryPalette: Sendable, Equatable {

Dory/Features/Settings/SettingsView.swift

@@ -864,8 +864,16 @@ struct SettingsView: View {
 
             groupLabel("APPEARANCE")
             HStack(spacing: 10) {
-                appearanceCard(.light, "Light", LinearGradient(colors: [Color(hex: 0xDCE9F7), .white], startPoint: .topLeading, endPoint: .bottomTrailing))
-                appearanceCard(.dark, "Dark", LinearGradient(colors: [Color(hex: 0x1B1D21), Color(hex: 0x2A2C33)], startPoint: .topLeading, endPoint: .bottomTrailing))
+                appearanceCard(.light, "Always light", LinearGradient(colors: [Color(hex: 0xDCE9F7), .white], startPoint: .topLeading, endPoint: .bottomTrailing))
+                appearanceCard(.dark, "Always dark", LinearGradient(colors: [Color(hex: 0x1B1D21), Color(hex: 0x2A2C33)], startPoint: .topLeading, endPoint: .bottomTrailing))
+                appearanceCard(.system, "Follows macOS", LinearGradient(
+                    stops: [
+                        .init(color: Color(hex: 0xDCE9F7), location: 0.5),
+                        .init(color: Color(hex: 0x1B1D21), location: 0.5)
+                    ],
+                    startPoint: .topLeading,
+                    endPoint: .bottomTrailing
+                ))
             }
         }
     }
@@ -949,13 +957,16 @@ struct SettingsView: View {
         .overlay(alignment: .bottom) { if divider { Rectangle().fill(p.border).frame(height: 1) } }
     }
 
-    private func appearanceCard(_ appearance: DoryAppearance, _ label: String, _ preview: LinearGradient) -> some View {
+    private func appearanceCard(_ appearance: DoryAppearance, _ subtitle: String, _ preview: LinearGradient) -> some View {
         let selected = store.appearance == appearance
         return Button { store.setAppearance(appearance) } label: {
             VStack(alignment: .leading, spacing: 9) {
                 RoundedRectangle(cornerRadius: 7).fill(preview).frame(height: 46)
                     .overlay(RoundedRectangle(cornerRadius: 7).strokeBorder(p.border))
-                Text(label).font(.system(size: 12.5, weight: .semibold)).foregroundStyle(p.text)
+                VStack(alignment: .leading, spacing: 1) {
+                    Text(appearance.label).font(.system(size: 12.5, weight: .semibold)).foregroundStyle(p.text)
+                    Text(subtitle).font(.system(size: 11)).foregroundStyle(p.text3)
+                }
             }
             .padding(13)
             .frame(maxWidth: .infinity)

Dory/Models/AppStore.swift

@@ -78,6 +78,7 @@ struct ContainerGroup: Identifiable, Sendable {
 @MainActor
 final class AppStore {
     var appearance: DoryAppearance = .dark
+    let systemAppearance = DorySystemAppearance.shared
     var section: AppSection = .containers {
         didSet { if oldValue != section { filter = "" } }
     }
@@ -391,7 +392,7 @@ final class AppStore {
         }
         if let raw = env["DORY_SETTINGS_TAB"], let parsed = SettingsTab(rawValue: raw) { settingsTab = parsed }
         if let raw = env["DORY_DETAIL_TAB"], let parsed = DetailTab(rawValue: raw) { detailTab = parsed }
-        if env["DORY_APPEARANCE"] == "light" { appearance = .light }
+        if let raw = env["DORY_APPEARANCE"], let parsed = DoryAppearance(rawValue: raw) { appearance = parsed }
         if let raw = env["DORY_SHEET"], let parsed = AppSheet(rawValue: raw) {
             activeSheet = parsed
             if parsed == .inspectImage { inspectedImage = images.first }
@@ -728,7 +729,7 @@ final class AppStore {
     func setAppearance(_ value: DoryAppearance) {
         appearance = value
         UserDefaults.standard.set(value.rawValue, forKey: Self.appearanceKey)
-        showSettingsSuccess("\(value.rawValue.capitalized) appearance applied.")
+        showSettingsSuccess(value == .system ? "Appearance now follows macOS." : "\(value.label) appearance applied.")
     }
 
     func setLaunchAtLogin(_ on: Bool) {
@@ -760,7 +761,11 @@ final class AppStore {
         UserDefaults.standard.set(true, forKey: Self.onboardingDoneKey)
     }
 
-    var palette: DoryPalette { appearance.palette }
+    var resolvedAppearance: DoryAppearance {
+        appearance.resolved(systemIsDark: systemAppearance.isDark)
+    }
+
+    var palette: DoryPalette { resolvedAppearance.palette }
 
     func clearSettingsNotice() {
         settingsNotice = nil
@@ -5306,7 +5311,7 @@ final class AppStore {
     }
 
     func toggleTheme() {
-        appearance = appearance == .dark ? .light : .dark
+        appearance = resolvedAppearance == .dark ? .light : .dark
     }
 
     @discardableResult

DoryTests/AppearanceTests.swift

@@ -0,0 +1,36 @@
+import Testing
+import SwiftUI
+@testable import Dory
+
+@MainActor
+struct AppearanceTests {
+    @Test func systemResolvesToTheOSAppearance() {
+        #expect(DoryAppearance.system.resolved(systemIsDark: true) == .dark)
+        #expect(DoryAppearance.system.resolved(systemIsDark: false) == .light)
+    }
+
+    @Test func explicitAppearancesIgnoreTheOSAppearance() {
+        #expect(DoryAppearance.light.resolved(systemIsDark: true) == .light)
+        #expect(DoryAppearance.dark.resolved(systemIsDark: false) == .dark)
+    }
+
+    @Test func systemLeavesTheColorSchemeToSwiftUI() {
+        #expect(DoryAppearance.system.colorScheme == nil)
+        #expect(DoryAppearance.light.colorScheme == .light)
+        #expect(DoryAppearance.dark.colorScheme == .dark)
+    }
+
+    @Test func paletteFollowsTheSystemAppearance() {
+        let store = AppStore()
+        store.appearance = .system
+        #expect(store.palette == (store.systemAppearance.isDark ? .dark : .light))
+    }
+
+    @Test func toggleThemeLeavesSystemForAnExplicitOverride() {
+        let store = AppStore()
+        store.appearance = .system
+        let wasDark = store.resolvedAppearance == .dark
+        store.toggleTheme()
+        #expect(store.appearance == (wasDark ? .light : .dark))
+    }
+}

DoryUITests/DoryScreensUITests.swift

@@ -69,8 +69,9 @@ final class DoryScreensUITests: XCTestCase {
 
     func testThemeToggleAndAppearancePicker() {
         nav("settings")
-        // Appearance picker selects light/dark without crashing.
+        // Appearance picker selects light/dark/system without crashing.
         app.buttons["appearance-light"].click()
+        app.buttons["appearance-system"].click()
         app.buttons["appearance-dark"].click()
         // The sidebar theme toggle flips appearance.
         let toggle = app.buttons["theme-toggle"]