Add the theme picker to the Display options panel

The previous version let you pick the theme from a dropdown on the Display
options page; the Avalonia port only exposed theme switching via the menu.
Bind a ComboBox to SessionSettings.Theme -- the same property the menu's
SetThemeCommand sets and ThemeManager applies live -- so the page switches
the theme too, and resetting Display defaults restores the default theme.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy.Tests/Options/ThemePickerTests.cs

@@ -0,0 +1,69 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System.Threading.Tasks;
+
+using Avalonia.Headless.NUnit;
+
+using AwesomeAssertions;
+
+using ILSpy;
+using ILSpy.AppEnv;
+using ILSpy.Options;
+using ILSpy.Themes;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// The Display options page exposes a theme picker bound to SessionSettings.Theme, which switches the
+/// theme live (ThemeManager applies it). (Regression: the picker was dropped from the Avalonia panel.)
+/// </summary>
+[TestFixture]
+public class ThemePickerTests
+{
+	[AvaloniaTest]
+	public async Task Display_Options_Theme_Picker_Lists_Themes_And_Switches_Live()
+	{
+		await TestHarness.BootAsync(1);
+		var settingsService = AppComposition.Current.GetExport<SettingsService>();
+		var original = settingsService.SessionSettings.Theme;
+		try
+		{
+			var vm = new DisplaySettingsViewModel();
+			vm.Load(settingsService);
+
+			vm.AllThemes.Should().Contain("Light").And.Contain("Dark");
+
+			// The picker binds two-way to the live SessionSettings.Theme -- the exact property
+			// SetThemeCommand sets and ThemeManager applies, so the picker switches the theme.
+			vm.SessionSettings.Should().BeSameAs(settingsService.SessionSettings);
+			vm.SessionSettings.Theme = "Dark";
+			settingsService.SessionSettings.Theme.Should().Be("Dark");
+
+			vm.LoadDefaults();
+			vm.SessionSettings.Theme.Should().Be(ThemeManager.Current.DefaultTheme,
+				"resetting Display defaults restores the default theme");
+		}
+		finally
+		{
+			settingsService.SessionSettings.Theme = original;
+		}
+	}
+}

ILSpy/Options/DisplaySettingsPanel.axaml

@@ -15,6 +15,11 @@
 	<ScrollViewer>
 		<Border Padding="6,6,6,12">
 			<StackPanel Spacing="10">
+			<StackPanel Orientation="Horizontal" Spacing="8">
+				<TextBlock VerticalAlignment="Center" Text="{x:Static res:Resources.DisplaySettingsPanel_Theme}" />
+				<ComboBox MinWidth="120" ItemsSource="{Binding AllThemes}"
+				          SelectedItem="{Binding SessionSettings.Theme, Mode=TwoWay}" />
+			</StackPanel>
 			<Grid ColumnDefinitions="Auto,*,Auto" RowDefinitions="Auto,Auto" ColumnSpacing="8" RowSpacing="4">
 				<TextBlock Grid.Row="0" Grid.Column="0" VerticalAlignment="Center"
 				           Text="{x:Static res:Resources.Font}" />

ILSpy/Options/DisplaySettingsViewModel.cs

@@ -40,6 +40,13 @@ namespace ILSpy.Options
 		[ObservableProperty]
 		DisplaySettings settings = null!;
 
+		// Session settings carry the active Theme. The theme ComboBox binds two-way to
+		// SessionSettings.Theme; ThemeManager already applies the change live on that property.
+		[ObservableProperty]
+		SessionSettings sessionSettings = null!;
+
+		public System.Collections.Generic.IReadOnlyCollection<string> AllThemes => Themes.ThemeManager.AllThemes;
+
 		// Avalonia equivalent of WPF's Fonts.SystemFontFamilies. FontManager.Current populates
 		// from the platform font enumerator (DirectWrite on Windows, FontConfig on Linux,
 		// CoreText on macOS), so the list is realistic on every supported target.
@@ -66,6 +73,7 @@ namespace ILSpy.Options
 				Settings.PropertyChanged -= OnSettingsPropertyChanged;
 			Settings = service.DisplaySettings;
 			Settings.PropertyChanged += OnSettingsPropertyChanged;
+			SessionSettings = service.SessionSettings;
 			OnPropertyChanged(nameof(CurrentFontFamily));
 		}
 
@@ -80,6 +88,7 @@ namespace ILSpy.Options
 			// Reset by replaying LoadFromXml against an empty element — every attribute is
 			// absent so each property falls back to its built-in default.
 			Settings.LoadFromXml(new XElement("DisplaySettings"));
+			SessionSettings.Theme = Themes.ThemeManager.Current.DefaultTheme;
 		}
 	}
 }