Fix #3994: theme the documentation hover popup's chrome

The rich hover popup hardcoded a near-white background and border while
its signature text is coloured by the active highlighting theme, so in
dark mode light-on-dark syntax colours landed on a light box and were
unreadable. The chrome and the doc-link colour now route through
theme-variant brushes; light mode keeps the established near-white look.

Assisted-by: Claude:claude-fable-5:Claude Code

ILSpy.Tests/Editor/DocumentationLinkTests.cs

@@ -80,6 +80,10 @@ public class DocumentationLinkTests
 			.FirstOrDefault(tb => tb.Classes.Contains("doc-link"));
 		link.Should().NotBeNull("a resolvable cref must render as a clickable link");
 		link!.Cursor.Should().NotBeNull("links show the hand cursor as a click affordance");
+		window.TryFindResource("ILSpy.DocLinkForeground", window.ActualThemeVariant, out var linkBrush)
+			.Should().BeTrue("doc links route their colour through a themed brush");
+		link.Foreground.Should().Be(linkBrush,
+			"a hardcoded link colour is unreadable on the dark popup background (issue #3994)");
 
 		object? navigated = null;
 		var linkClickedRaised = false;

ILSpy.Tests/Editor/DocumentationRendererThemeTests.cs

@@ -0,0 +1,88 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// 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 Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Media;
+using Avalonia.Styling;
+using Avalonia.Threading;
+
+using AvaloniaEdit.Highlighting;
+
+using AwesomeAssertions;
+
+using ICSharpCode.Decompiler.CSharp.OutputVisitor;
+
+using ICSharpCode.ILSpy.TextView;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.TextView;
+
+/// <summary>
+/// The hover popup's signature text is coloured by the active highlighting theme, so its
+/// chrome must follow the same theme variant: dark-theme (light-on-dark) text on a
+/// hardcoded near-white background is unreadable (issue #3994). Light mode keeps the
+/// established near-white look.
+/// </summary>
+[TestFixture]
+public class DocumentationRendererThemeTests
+{
+	[AvaloniaTest]
+	public void Popup_Chrome_Follows_The_Active_Theme_Variant()
+	{
+		var renderer = new DocumentationRenderer(
+			new CSharpAmbience(),
+			new FontFamily("Consolas, Menlo, Monospace"),
+			12);
+		renderer.AddSignatureBlock(new RichText("(parameter) string matchText"));
+
+		var view = (Border)renderer.CreateView();
+		var window = new Window { Content = view };
+		var app = Application.Current!;
+		var originalVariant = app.RequestedThemeVariant;
+		try
+		{
+			window.Show();
+
+			app.RequestedThemeVariant = ThemeVariant.Dark;
+			Dispatcher.UIThread.RunJobs();
+
+			window.TryFindResource("ILSpy.DocTooltipBackground", ThemeVariant.Dark, out var darkBackground)
+				.Should().BeTrue("the popup chrome must route through themed brushes");
+			view.Background.Should().Be(darkBackground,
+				"dark-theme signature colours are unreadable on a light popup background");
+			window.TryFindResource("ILSpy.DocTooltipBorder", ThemeVariant.Dark, out var darkBorder)
+				.Should().BeTrue();
+			view.BorderBrush.Should().Be(darkBorder);
+
+			app.RequestedThemeVariant = ThemeVariant.Light;
+			Dispatcher.UIThread.RunJobs();
+
+			((ISolidColorBrush)view.Background!).Color.Should().Be(Color.FromRgb(0xFC, 0xFC, 0xFC),
+				"light mode keeps the established near-white popup chrome");
+			((ISolidColorBrush)view.BorderBrush!).Color.Should().Be(Color.FromRgb(0xAA, 0xAA, 0xAA));
+		}
+		finally
+		{
+			window.Close();
+			app.RequestedThemeVariant = originalVariant;
+		}
+	}
+}

ILSpy/App.axaml

@@ -60,6 +60,12 @@
                     <SolidColorBrush x:Key="ILSpy.TooltipBackground" Color="#FFFFE1" />
                     <SolidColorBrush x:Key="ILSpy.TooltipForeground" Color="Black" />
                     <SolidColorBrush x:Key="ILSpy.TooltipBorder" Color="#FF767676" />
+                    <!-- Documentation hover popup (signature + XML docs). Kept separate from the
+                         plain ILSpy.Tooltip* brushes: the doc popup uses a near-white surface for
+                         syntax-coloured code, not the classic yellow tooltip fill. -->
+                    <SolidColorBrush x:Key="ILSpy.DocTooltipBackground" Color="#FCFCFC" />
+                    <SolidColorBrush x:Key="ILSpy.DocTooltipBorder" Color="#AAAAAA" />
+                    <SolidColorBrush x:Key="ILSpy.DocLinkForeground" Color="#0066CC" />
                     <SolidColorBrush x:Key="ILSpy.WindowBackground" Color="#F0F0F0" />
                     <SolidColorBrush x:Key="ILSpy.WindowForeground" Color="Black" />
                     <SolidColorBrush x:Key="ILSpy.PaneBackground" Color="White" />
@@ -123,6 +129,12 @@
                     <SolidColorBrush x:Key="ILSpy.TooltipBackground" Color="#252526" />
                     <SolidColorBrush x:Key="ILSpy.TooltipForeground" Color="#DCDCDC" />
                     <SolidColorBrush x:Key="ILSpy.TooltipBorder" Color="#3F3F46" />
+                    <!-- Dark counterpart of the doc hover popup: slightly lighter than the #1E1E1E
+                         editor canvas so the popup reads as a raised surface, with the link blue
+                         brightened for contrast on the dark fill. -->
+                    <SolidColorBrush x:Key="ILSpy.DocTooltipBackground" Color="#252526" />
+                    <SolidColorBrush x:Key="ILSpy.DocTooltipBorder" Color="#3F3F46" />
+                    <SolidColorBrush x:Key="ILSpy.DocLinkForeground" Color="#6FA8DC" />
                     <SolidColorBrush x:Key="ILSpy.WindowBackground" Color="#252526" />
                     <SolidColorBrush x:Key="ILSpy.WindowForeground" Color="#DCDCDC" />
                     <SolidColorBrush x:Key="ILSpy.PaneBackground" Color="#1E1E1E" />
@@ -237,6 +249,12 @@
             <Setter Property="BorderThickness" Value="1" />
         </Style>
 
+        <!-- Links inside the documentation hover popup (DocumentationRenderer tags them with
+             the doc-link class). Colour lives here so it swaps with the theme variant. -->
+        <Style Selector="TextBlock.doc-link">
+            <Setter Property="Foreground" Value="{DynamicResource ILSpy.DocLinkForeground}" />
+        </Style>
+
         <!-- Window-level chrome: every top-level window picks up these brushes for the
              canvas behind the menu / toolbar / dock area. Foreground propagates by
              inheritance so default-styled text inside the window goes light on dark. -->

ILSpy/TextView/DocumentationRenderer.cs

@@ -48,8 +48,6 @@ namespace ICSharpCode.ILSpy.TextView
 	/// </summary>
 	public sealed class DocumentationRenderer
 	{
-		static readonly IBrush HyperlinkBrush = new SolidColorBrush(Color.FromRgb(0x00, 0x66, 0xCC));
-
 		readonly IAmbience ambience;
 		readonly FontFamily codeFont;
 		readonly double fontSize;
@@ -118,14 +116,18 @@ namespace ICSharpCode.ILSpy.TextView
 				VerticalScrollBarVisibility = ScrollBarVisibility.Auto,
 				MaxHeight = maxHeight,
 			};
-			return new Border {
+			var border = new Border {
 				BorderThickness = new Thickness(1),
-				BorderBrush = new SolidColorBrush(Color.FromRgb(0xAA, 0xAA, 0xAA)),
-				Background = new SolidColorBrush(Color.FromRgb(0xFC, 0xFC, 0xFC)),
 				Padding = new Thickness(6),
 				MaxWidth = maxWidth,
 				Child = scroll,
 			};
+			// The signature text is coloured by the active highlighting theme, so the chrome
+			// must follow the same theme variant: dark-theme text on a fixed light fill is
+			// unreadable. DynamicResource-style bindings keep it in sync on theme switches.
+			border.Bind(Border.BackgroundProperty, border.GetResourceObservable("ILSpy.DocTooltipBackground"));
+			border.Bind(Border.BorderBrushProperty, border.GetResourceObservable("ILSpy.DocTooltipBorder"));
+			return border;
 		}
 
 		public void AddSignatureBlock(RichText signature)
@@ -392,8 +394,8 @@ namespace ICSharpCode.ILSpy.TextView
 		// embedded TextBlock inside an InlineUIContainer.
 		static TextBlock CreateLinkTextBlock()
 		{
+			// Foreground comes from the App.axaml "doc-link" style so it follows the theme.
 			return new TextBlock {
-				Foreground = HyperlinkBrush,
 				TextDecorations = TextDecorations.Underline,
 				Cursor = new Cursor(StandardCursorType.Hand),
 				// A null background makes the TextBlock hit-test invisible — clicks would