diff --git a/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Dark/DiffHighlightingDefinition.cs b/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Dark/DiffHighlightingDefinition.cs
new file mode 100644
index 0000000..52e6ed7
--- /dev/null
+++ b/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Dark/DiffHighlightingDefinition.cs
@@ -0,0 +1,124 @@
+// Copyright © 2017-2025 QL-Win Contributors
+//
+// This file is part of QuickLook program.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+using ICSharpCode.AvalonEdit.Document;
+using ICSharpCode.AvalonEdit.Highlighting;
+using ICSharpCode.AvalonEdit.Rendering;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+
+namespace QuickLook.Plugin.TextViewer.Themes.HighlightingDefinitions.Dark;
+
+public class DiffHighlightingDefinition : DarkHighlightingDefinition
+{
+    public override string Name => "Diff";
+
+    public override string Extension => ".diff";
+
+    public override HighlightingRuleSet MainRuleSet => new()
+    {
+        Rules =
+        {
+            // Diff header (diff --git, index, etc.)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^(diff --git|index|---|\+\+\+)", RegexOptions.Compiled),
+                Color = GetNamedColor("Header")
+            },
+            // File mode and other metadata
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^(new file mode|deleted file mode|old mode|new mode|similarity index|dissimilarity index|rename from|rename to|copy from|copy to)", RegexOptions.Compiled),
+                Color = GetNamedColor("Metadata")
+            },
+            // Hunk headers (@@ ... @@)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^@@.*@@$", RegexOptions.Compiled),
+                Color = GetNamedColor("HunkHeader")
+            },
+            // Added lines (+)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^\+.*$", RegexOptions.Compiled),
+                Color = GetNamedColor("Added")
+            },
+            // Removed lines (-)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^-.*$", RegexOptions.Compiled),
+                Color = GetNamedColor("Removed")
+            },
+            // Context lines (unchanged)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^ .*", RegexOptions.Compiled),
+                Color = GetNamedColor("Context")
+            }
+        }
+    };
+
+    public override HighlightingColor GetNamedColor(string name)
+    {
+        return name switch
+        {
+            "Header" => new HighlightingColor
+            {
+                Name = "Header",
+                Foreground = new SimpleHighlightingBrush("#569CD6".ToColor()), // Blue for headers
+            },
+            "Metadata" => new HighlightingColor
+            {
+                Name = "Metadata",
+                Foreground = new SimpleHighlightingBrush("#808080".ToColor()), // Gray for metadata
+            },
+            "HunkHeader" => new HighlightingColor
+            {
+                Name = "HunkHeader",
+                Foreground = new SimpleHighlightingBrush("#C586C0".ToColor()), // Purple for hunk headers
+            },
+            "Added" => new HighlightingColor
+            {
+                Name = "Added",
+                Foreground = new SimpleHighlightingBrush("#6A9949".ToColor()), // Green for added lines
+            },
+            "Removed" => new HighlightingColor
+            {
+                Name = "Removed",
+                Foreground = new SimpleHighlightingBrush("#F44747".ToColor()), // Red for removed lines
+            },
+            "Context" => new HighlightingColor
+            {
+                Name = "Context",
+                Foreground = new SimpleHighlightingBrush("#D4D4D4".ToColor()), // Light gray for context
+            },
+            _ => null,
+        };
+    }
+
+    public override IEnumerable<HighlightingColor> NamedHighlightingColors =>
+    [
+        GetNamedColor("Header"),
+        GetNamedColor("Metadata"),
+        GetNamedColor("HunkHeader"),
+        GetNamedColor("Added"),
+        GetNamedColor("Removed"),
+        GetNamedColor("Context"),
+    ];
+
+    public override DocumentColorizingTransformer[] LineTransformers { get; } = [];
+}
diff --git a/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Light/DiffHighlightingDefinition.cs b/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Light/DiffHighlightingDefinition.cs
new file mode 100644
index 0000000..0b34ac5
--- /dev/null
+++ b/QuickLook.Plugin/QuickLook.Plugin.TextViewer/Themes/HighlightingDefinitions/Light/DiffHighlightingDefinition.cs
@@ -0,0 +1,124 @@
+// Copyright © 2017-2025 QL-Win Contributors
+//
+// This file is part of QuickLook program.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+using ICSharpCode.AvalonEdit.Document;
+using ICSharpCode.AvalonEdit.Highlighting;
+using ICSharpCode.AvalonEdit.Rendering;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+
+namespace QuickLook.Plugin.TextViewer.Themes.HighlightingDefinitions.Light;
+
+public class DiffHighlightingDefinition : LightHighlightingDefinition
+{
+    public override string Name => "Diff";
+
+    public override string Extension => ".diff";
+
+    public override HighlightingRuleSet MainRuleSet => new()
+    {
+        Rules =
+        {
+            // Diff header (diff --git, index, etc.)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^(diff --git|index|---|\+\+\+)", RegexOptions.Compiled),
+                Color = GetNamedColor("Header")
+            },
+            // File mode and other metadata
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^(new file mode|deleted file mode|old mode|new mode|similarity index|dissimilarity index|rename from|rename to|copy from|copy to)", RegexOptions.Compiled),
+                Color = GetNamedColor("Metadata")
+            },
+            // Hunk headers (@@ ... @@)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^@@.*@@$", RegexOptions.Compiled),
+                Color = GetNamedColor("HunkHeader")
+            },
+            // Added lines (+)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^\+.*$", RegexOptions.Compiled),
+                Color = GetNamedColor("Added")
+            },
+            // Removed lines (-)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^-.*$", RegexOptions.Compiled),
+                Color = GetNamedColor("Removed")
+            },
+            // Context lines (unchanged)
+            new HighlightingRule
+            {
+                Regex = new Regex(@"^ .*", RegexOptions.Compiled),
+                Color = GetNamedColor("Context")
+            }
+        }
+    };
+
+    public override HighlightingColor GetNamedColor(string name)
+    {
+        return name switch
+        {
+            "Header" => new HighlightingColor
+            {
+                Name = "Header",
+                Foreground = new SimpleHighlightingBrush("#2563EB".ToColor()), // Blue for headers
+            },
+            "Metadata" => new HighlightingColor
+            {
+                Name = "Metadata",
+                Foreground = new SimpleHighlightingBrush("#6B7280".ToColor()), // Gray for metadata
+            },
+            "HunkHeader" => new HighlightingColor
+            {
+                Name = "HunkHeader",
+                Foreground = new SimpleHighlightingBrush("#7C3AED".ToColor()), // Purple for hunk headers
+            },
+            "Added" => new HighlightingColor
+            {
+                Name = "Added",
+                Foreground = new SimpleHighlightingBrush("#15803D".ToColor()), // Green for added lines
+            },
+            "Removed" => new HighlightingColor
+            {
+                Name = "Removed",
+                Foreground = new SimpleHighlightingBrush("#DC2626".ToColor()), // Red for removed lines
+            },
+            "Context" => new HighlightingColor
+            {
+                Name = "Context",
+                Foreground = new SimpleHighlightingBrush("#374151".ToColor()), // Gray for context
+            },
+            _ => null,
+        };
+    }
+
+    public override IEnumerable<HighlightingColor> NamedHighlightingColors =>
+    [
+        GetNamedColor("Header"),
+        GetNamedColor("Metadata"),
+        GetNamedColor("HunkHeader"),
+        GetNamedColor("Added"),
+        GetNamedColor("Removed"),
+        GetNamedColor("Context"),
+    ];
+
+    public override DocumentColorizingTransformer[] LineTransformers { get; } = [];
+}