diff --git a/HellionChat/Themes/ThemeJsonWriter.cs b/HellionChat/Themes/ThemeJsonWriter.cs
new file mode 100644
index 0000000..97523f1
--- /dev/null
+++ b/HellionChat/Themes/ThemeJsonWriter.cs
@@ -0,0 +1,65 @@
+using System.Text.Json;
+
+namespace HellionChat.Themes;
+
+internal static class ThemeJsonWriter
+{
+    public static string Serialize(Theme theme)
+    {
+        using var ms = new MemoryStream();
+        using (var writer = new Utf8JsonWriter(ms, new JsonWriterOptions { Indented = true }))
+        {
+            writer.WriteStartObject();
+            writer.WriteNumber("schemaVersion", ThemeJsonLoader.SupportedSchemaVersion);
+            writer.WriteString("slug", theme.Slug);
+            writer.WriteString("name", theme.Name);
+            writer.WriteString("author", theme.Author);
+            writer.WriteString("description", theme.Description);
+
+            writer.WriteStartObject("colors");
+            WriteColor(writer, "primaryDark",   theme.Colors.PrimaryDark);
+            WriteColor(writer, "primary",       theme.Colors.Primary);
+            WriteColor(writer, "primaryLight",  theme.Colors.PrimaryLight);
+            WriteColor(writer, "primaryGlow",   theme.Colors.PrimaryGlow);
+            WriteColor(writer, "accentDark",    theme.Colors.AccentDark);
+            WriteColor(writer, "accent",        theme.Colors.Accent);
+            WriteColor(writer, "accentLight",   theme.Colors.AccentLight);
+            WriteColor(writer, "identity",      theme.Colors.Identity);
+            WriteColor(writer, "windowBg",      theme.Colors.WindowBg);
+            WriteColor(writer, "childBg",       theme.Colors.ChildBg);
+            WriteColor(writer, "frameBg",       theme.Colors.FrameBg);
+            WriteColor(writer, "surface",       theme.Colors.Surface);
+            WriteColor(writer, "surfaceHover",  theme.Colors.SurfaceHover);
+            WriteColor(writer, "border",        theme.Colors.Border);
+            WriteColor(writer, "textPrimary",   theme.Colors.TextPrimary);
+            WriteColor(writer, "textMuted",     theme.Colors.TextMuted);
+            WriteColor(writer, "textDim",       theme.Colors.TextDim);
+            WriteColor(writer, "statusSuccess", theme.Colors.StatusSuccess);
+            WriteColor(writer, "statusDanger",  theme.Colors.StatusDanger);
+            WriteColor(writer, "statusWarning", theme.Colors.StatusWarning);
+            WriteColor(writer, "statusInfo",    theme.Colors.StatusInfo);
+            writer.WriteEndObject();
+
+            writer.WriteStartObject("layout");
+            writer.WriteNumber("windowRounding",    theme.Layout.WindowRounding);
+            writer.WriteNumber("childRounding",     theme.Layout.ChildRounding);
+            writer.WriteNumber("popupRounding",     theme.Layout.PopupRounding);
+            writer.WriteNumber("frameRounding",     theme.Layout.FrameRounding);
+            writer.WriteNumber("grabRounding",      theme.Layout.GrabRounding);
+            writer.WriteNumber("tabRounding",       theme.Layout.TabRounding);
+            writer.WriteNumber("scrollbarRounding", theme.Layout.ScrollbarRounding);
+            writer.WriteNumber("windowBorderSize",  theme.Layout.WindowBorderSize);
+            writer.WriteNumber("frameBorderSize",   theme.Layout.FrameBorderSize);
+            writer.WriteEndObject();
+
+            writer.WriteEndObject();
+        }
+
+        return System.Text.Encoding.UTF8.GetString(ms.ToArray());
+    }
+
+    private static void WriteColor(Utf8JsonWriter writer, string key, uint rgba)
+    {
+        writer.WriteString(key, $"#{rgba:X8}");
+    }
+}
diff --git a/HellionChat/Ui/SettingsTabs/Themes.cs b/HellionChat/Ui/SettingsTabs/Themes.cs
index 0fe85af..f04a5de 100644
--- a/HellionChat/Ui/SettingsTabs/Themes.cs
+++ b/HellionChat/Ui/SettingsTabs/Themes.cs
@@ -67,7 +67,13 @@ internal sealed class Themes : ISettingsTab
         var exportLabel = HellionStrings.ResourceManager.GetString("Settings_Themes_ExportActive") ?? "Export active...";
         if (ImGui.Button(exportLabel))
         {
-            // Export-Logik wird in Phase L (Task 21) ergänzt — Stub belassen, Button bleibt sichtbar.
+            var dir = Path.Combine(Plugin.Interface.ConfigDirectory.FullName, "themes");
+            Directory.CreateDirectory(dir);
+            var fileName = $"{active.Slug}.export.json";
+            var path = Path.Combine(dir, fileName);
+            var json = ThemeJsonWriter.Serialize(active);
+            File.WriteAllText(path, json);
+            Plugin.Log.Information($"Exported active theme '{active.Slug}' to {path}");
         }
     }