diff --git a/ILSpy.Tests/ThemeAwareHighlightingColorizerTests.cs b/ILSpy.Tests/ThemeAwareHighlightingColorizerTests.cs
new file mode 100644
index 000000000..05db23704
--- /dev/null
+++ b/ILSpy.Tests/ThemeAwareHighlightingColorizerTests.cs
@@ -0,0 +1,140 @@
+// 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 System.Collections.Generic;
+
+using Avalonia.Headless.NUnit;
+using Avalonia.Media;
+
+using AvaloniaEdit.Highlighting;
+using AvaloniaEdit.Rendering;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpy;
+using ICSharpCode.ILSpy.TextView;
+using ICSharpCode.ILSpy.Themes;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// Guards the ThemeManager / ThemeAwareHighlightingColorizer contract against double dark
+/// conversion. ThemeManager darkens a REGISTERED definition's named colours in place; the
+/// colorizer per-paint-remaps colours of UNREGISTERED definitions. Both mechanisms active at
+/// once on the same definition converts colours twice (washed-out output), so the colorizer
+/// must observe a registration that happens after it was constructed, and must not serve
+/// cached conversions computed from colour values that a theme switch has since replaced.
+/// </summary>
+[TestFixture]
+public class ThemeAwareHighlightingColorizerTests
+{
+	// Minimal definition stub: real Properties (ThemeManager marks theme-awareness there)
+	// and real named colours (registration snapshots and rewrites them in place).
+	sealed class StubHighlightingDefinition : IHighlightingDefinition
+	{
+		readonly List<HighlightingColor> colors;
+
+		public StubHighlightingDefinition(params HighlightingColor[] colors)
+		{
+			this.colors = new List<HighlightingColor>(colors);
+		}
+
+		public string Name => "ColorizerTestStub";
+		public HighlightingRuleSet MainRuleSet { get; } = new();
+		public IEnumerable<HighlightingColor> NamedHighlightingColors => colors;
+		public IDictionary<string, string> Properties { get; } = new Dictionary<string, string>();
+		public HighlightingRuleSet GetNamedRuleSet(string name) => MainRuleSet;
+		public HighlightingColor GetNamedColor(string name) => colors.Find(c => c.Name == name)!;
+	}
+
+	static HighlightingColor MakeColor(string name, Color foreground)
+		=> new() { Name = name, Foreground = new SimpleHighlightingBrush(foreground) };
+
+	// Drives ThemeManager through its public surface; restores Light afterwards so the
+	// process-lived singleton doesn't leak dark mode into other tests.
+	static SessionSettings AttachThemeSettings()
+	{
+		var settings = new SessionSettings();
+		ThemeManager.Current.Attach(settings);
+		return settings;
+	}
+
+	[AvaloniaTest]
+	public void RegistrationAfterConstructionDisablesPerPaintRemap()
+	{
+		var settings = AttachThemeSettings();
+		try
+		{
+			var color = MakeColor("Keyword", Colors.Blue);
+			var definition = new StubHighlightingDefinition(color);
+			var colorizer = new ThemeAwareHighlightingColorizer(definition);
+
+			settings.Theme = "Dark";
+
+			// Unregistered definition: the colorizer owns the dark conversion.
+			colorizer.GetEffectiveColor(color).Should().NotBeSameAs(color,
+				"an unregistered definition's colours must be remapped per paint in dark mode");
+
+			// Late registration: ThemeManager now darkens the live colours in place. If the
+			// colorizer kept remapping on top of that, every colour would be converted twice.
+			ThemeManager.Current.RegisterThemableDefinition(definition);
+
+			colorizer.GetEffectiveColor(color).Should().BeSameAs(color,
+				"once the definition is theme-managed, a second per-paint conversion would double-darken");
+		}
+		finally
+		{
+			settings.Theme = "Light";
+		}
+	}
+
+	[AvaloniaTest]
+	public void DarkCacheDoesNotSurviveThemeSwitch()
+	{
+		var settings = AttachThemeSettings();
+		try
+		{
+			var color = MakeColor("String", Colors.Red);
+			var definition = new StubHighlightingDefinition(color);
+			var colorizer = new ThemeAwareHighlightingColorizer(definition);
+
+			settings.Theme = "Dark";
+			var beforeSwitch = colorizer.GetEffectiveColor(color);
+
+			// The colour's content changes in place (as ThemeManager does for managed
+			// definitions) with no paint in between. The colorizer's conversion cache is
+			// keyed by HighlightingColor's content-based equality, so the changed content
+			// must miss the cache and reconvert -- serving the conversion of the old values
+			// here would paint stale colours. If HighlightingColor ever moved to reference
+			// equality, this test goes red and the cache needs an explicit flush instead.
+			color.Foreground = new SimpleHighlightingBrush(Colors.Lime);
+			settings.Theme = "Light";
+			settings.Theme = "Dark";
+
+			var afterSwitch = colorizer.GetEffectiveColor(color);
+			afterSwitch.Should().NotBeSameAs(beforeSwitch,
+				"conversions cached from superseded colour values must not be served");
+		}
+		finally
+		{
+			settings.Theme = "Light";
+		}
+	}
+}
diff --git a/ILSpy/TextView/ThemeAwareHighlightingColorizer.cs b/ILSpy/TextView/ThemeAwareHighlightingColorizer.cs
index 34d13db51..a34c13d61 100644
--- a/ILSpy/TextView/ThemeAwareHighlightingColorizer.cs
+++ b/ILSpy/TextView/ThemeAwareHighlightingColorizer.cs
@@ -27,27 +27,40 @@ namespace ICSharpCode.ILSpy.TextView
 {
 	/// <summary>
 	/// Wraps AvaloniaEdit's default colorizer so colours flip when the active theme is
-	/// Dark. The decision is per-paint and reads <see cref="ThemeManager.IsDarkTheme"/>
-	/// directly, so a theme switch followed by an editor redraw renders in the new
-	/// palette without needing a second colorizer instance. The remapped colours are
-	/// cached per source <see cref="HighlightingColor"/> instance.
+	/// Dark. The whole decision is per-paint: it reads <see cref="ThemeManager.IsDarkTheme"/>
+	/// and <see cref="ThemeManager.IsThemeAware"/> directly, so a theme switch -- or a
+	/// definition getting registered with the theme manager after this colorizer was
+	/// created -- takes effect on the next redraw without needing a second colorizer
+	/// instance. Checking theme-awareness per paint matters for correctness, not just
+	/// freshness: <see cref="ThemeManager"/> darkens a registered definition's named
+	/// colours in place, so remapping them here a second time would wash the palette out.
+	/// The remapped colours are cached per source <see cref="HighlightingColor"/>; the
+	/// cache key is content-based (HighlightingColor overrides Equals/GetHashCode), so an
+	/// in-place recolour of a source colour misses the cache and reconverts instead of
+	/// serving a conversion of the old values.
 	/// </summary>
 	public sealed class ThemeAwareHighlightingColorizer : HighlightingColorizer
 	{
 		readonly Dictionary<HighlightingColor, HighlightingColor> darkColors = new();
-		readonly bool definitionIsThemeAware;
+		readonly IHighlightingDefinition definition;
 
 		public ThemeAwareHighlightingColorizer(IHighlightingDefinition highlightingDefinition)
 			: base(highlightingDefinition)
 		{
-			definitionIsThemeAware = ThemeManager.Current.IsThemeAware(highlightingDefinition);
+			definition = highlightingDefinition;
 		}
 
 		protected override void ApplyColorToElement(VisualLineElement element, HighlightingColor color)
 		{
-			if (!definitionIsThemeAware && ThemeManager.Current.IsDarkTheme)
-				color = GetCachedDarkColor(color);
-			base.ApplyColorToElement(element, color);
+			base.ApplyColorToElement(element, GetEffectiveColor(color));
+		}
+
+		// The per-paint colour decision, split out so tests can drive it without a TextView.
+		internal HighlightingColor GetEffectiveColor(HighlightingColor color)
+		{
+			if (ThemeManager.Current.IsDarkTheme && !ThemeManager.Current.IsThemeAware(definition))
+				return GetCachedDarkColor(color);
+			return color;
 		}
 
 		HighlightingColor GetCachedDarkColor(HighlightingColor lightColor)