feat(ui): add Halloween, Christmas and 4th of July color schemes

Built-in festive schemes theme the background, sheet and accents while
keeping 12-color part palettes distinct, light enough for black part IDs
and clear of the green etch stroke. They are only applied when chosen;
there is no automatic seasonal switching.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.WinForms.Tests/FestiveColorSchemeTests.cs

@@ -0,0 +1,56 @@
+using System.Drawing;
+
+namespace OpenNest.WinForms.Tests;
+
+public class FestiveColorSchemeTests
+{
+    [Theory]
+    [InlineData("Halloween")]
+    [InlineData("Christmas")]
+    [InlineData("4th of July")]
+    public void FestiveSchemeKeepsPartsDistinctAndReadable(string name)
+    {
+        var scheme = ColorSchemeRegistry.Get(name);
+
+        Assert.Equal(name, scheme.Name);
+        Assert.False(scheme.UseGoldenAngleColors);
+        Assert.True(scheme.PartColors.Length >= 12);
+        Assert.Equal(Color.Lime.ToArgb(), scheme.EtchColor.ToArgb());
+        Assert.True(scheme.SelectedPartColor.IsEmpty);
+
+        var colors = scheme.PartColors;
+        for (var i = 0; i < colors.Length; i++)
+        {
+            // Part IDs are drawn in black over the fill.
+            Assert.True(Luminance(colors[i]) >= 90, $"{name} part color {i} is too dark for black part IDs.");
+            Assert.True(Distance(colors[i], scheme.EtchColor) >= 120, $"{name} part color {i} is too close to the etch color.");
+            for (var j = i + 1; j < colors.Length; j++)
+                Assert.True(Distance(colors[i], colors[j]) >= 40, $"{name} part colors {i} and {j} are too similar.");
+        }
+    }
+
+    [Fact]
+    public void FestiveSchemesRoundTripAndLeaveClassicAsDefault()
+    {
+        foreach (var name in new[] { "Halloween", "Christmas", "4th of July" })
+        {
+            var scheme = ColorSchemeRegistry.Get(name);
+            var restored = ColorSchemeSerializer.Deserialize(ColorSchemeSerializer.Serialize(scheme));
+            Assert.Equal(name, restored.Name);
+            Assert.Equal(scheme.BackgroundColor.ToArgb(), restored.BackgroundColor.ToArgb());
+            Assert.Equal(scheme.PartColors.Select(c => c.ToArgb()), restored.PartColors.Select(c => c.ToArgb()));
+        }
+
+        Assert.Equal("Classic", ColorSchemeRegistry.Get(null).Name);
+    }
+
+    private static double Distance(Color a, Color b)
+    {
+        var dr = a.R - b.R;
+        var dg = a.G - b.G;
+        var db = a.B - b.B;
+        return System.Math.Sqrt(dr * dr + dg * dg + db * db);
+    }
+
+    private static double Luminance(Color c) => 0.299 * c.R + 0.587 * c.G + 0.114 * c.B;
+}

OpenNest/ColorSchemeRegistry.cs

@@ -19,6 +19,9 @@ namespace OpenNest
             ["Pastel"] = BuildPastel(),
             ["Dark"] = BuildDark(),
             ["Workshop"] = BuildWorkshop(),
+            ["Halloween"] = BuildHalloween(),
+            ["Christmas"] = BuildChristmas(),
+            ["4th of July"] = BuildFourthOfJuly(),
         };
 
         private static List<ColorScheme> diskCache;
@@ -135,6 +138,95 @@ namespace OpenNest
                 PartColors = Enumerable.Range(0, 12).Select(PartColorPalette.GoldenAngle).ToArray(),
             };
 
+        // Festive schemes theme the background, sheet and accents; their part palettes
+        // stay varied and light enough for black part IDs, and avoid the etch green.
+        private static ColorScheme BuildHalloween() =>
+            new ColorScheme
+            {
+                Name = "Halloween",
+                BackgroundColor = Color.FromArgb(28, 18, 36),
+                LayoutOutlineColor = Color.FromArgb(110, 80, 130),
+                LayoutFillColor = Color.FromArgb(52, 48, 58),
+                BoundingBoxColor = Color.FromArgb(170, 120, 230),
+                RapidColor = Color.FromArgb(255, 140, 0),
+                OriginColor = Color.FromArgb(130, 110, 140),
+                EdgeSpacingColor = Color.FromArgb(80, 70, 92),
+                PreviewPartColor = Color.FromArgb(160, 255, 80),
+                PartColors = new[]
+                {
+                    Color.FromArgb(255, 130, 30), // Pumpkin
+                    Color.FromArgb(170, 110, 230), // Witch Purple
+                    Color.FromArgb(255, 205, 60), // Candy Corn
+                    Color.FromArgb(235, 235, 245), // Ghost
+                    Color.FromArgb(220, 60, 60), // Blood Red
+                    Color.FromArgb(130, 170, 230), // Moonlight
+                    Color.FromArgb(225, 210, 170), // Bone
+                    Color.FromArgb(230, 90, 200), // Potion
+                    Color.FromArgb(60, 200, 180), // Cauldron Teal
+                    Color.FromArgb(200, 120, 60), // Caramel Apple
+                    Color.FromArgb(200, 170, 240), // Phantom Lavender
+                    Color.FromArgb(255, 90, 40), // Ember
+                },
+            };
+
+        private static ColorScheme BuildChristmas() =>
+            new ColorScheme
+            {
+                Name = "Christmas",
+                BackgroundColor = Color.FromArgb(18, 58, 38),
+                LayoutOutlineColor = Color.FromArgb(190, 160, 80),
+                LayoutFillColor = Color.FromArgb(248, 248, 244),
+                BoundingBoxColor = Color.FromArgb(200, 40, 50),
+                RapidColor = Color.FromArgb(200, 150, 30),
+                OriginColor = Color.FromArgb(150, 130, 90),
+                EdgeSpacingColor = Color.FromArgb(215, 220, 215),
+                PreviewPartColor = Color.FromArgb(230, 60, 60),
+                PartColors = new[]
+                {
+                    Color.FromArgb(215, 60, 60), // Holly Red
+                    Color.FromArgb(80, 160, 90), // Pine
+                    Color.FromArgb(230, 190, 70), // Gold
+                    Color.FromArgb(150, 200, 235), // Ice
+                    Color.FromArgb(190, 195, 205), // Silver
+                    Color.FromArgb(240, 150, 160), // Candy Cane
+                    Color.FromArgb(175, 120, 200), // Sugarplum
+                    Color.FromArgb(150, 215, 170), // Mint
+                    Color.FromArgb(190, 110, 70), // Cinnamon
+                    Color.FromArgb(180, 50, 80), // Cranberry
+                    Color.FromArgb(245, 225, 160), // Eggnog
+                    Color.FromArgb(100, 150, 200), // Winter Sky
+                },
+            };
+
+        private static ColorScheme BuildFourthOfJuly() =>
+            new ColorScheme
+            {
+                Name = "4th of July",
+                BackgroundColor = Color.FromArgb(18, 28, 60),
+                LayoutOutlineColor = Color.FromArgb(110, 120, 150),
+                LayoutFillColor = Color.FromArgb(250, 250, 252),
+                BoundingBoxColor = Color.FromArgb(120, 150, 230),
+                RapidColor = Color.FromArgb(210, 40, 50),
+                OriginColor = Color.FromArgb(120, 130, 160),
+                EdgeSpacingColor = Color.FromArgb(205, 210, 225),
+                PreviewPartColor = Color.FromArgb(255, 190, 40),
+                PartColors = new[]
+                {
+                    Color.FromArgb(220, 60, 60), // Old Glory Red
+                    Color.FromArgb(110, 160, 230), // Liberty Blue
+                    Color.FromArgb(250, 195, 60), // Firework Gold
+                    Color.FromArgb(200, 205, 215), // Star Silver
+                    Color.FromArgb(220, 90, 180), // Magenta Burst
+                    Color.FromArgb(70, 190, 180), // Teal Burst
+                    Color.FromArgb(240, 120, 110), // Coral
+                    Color.FromArgb(150, 200, 240), // Sky
+                    Color.FromArgb(250, 150, 60), // Sparkler
+                    Color.FromArgb(170, 150, 230), // Twilight
+                    Color.FromArgb(235, 150, 170), // Rose
+                    Color.FromArgb(90, 120, 210), // Royal Blue
+                },
+            };
+
         private static ColorScheme BuildClassic() =>
             new ColorScheme
             {

docs/nesting-view-colors.md

@@ -36,6 +36,14 @@ classified as `Cut` remains a cut line. Drawing geometry, layer classifications,
 cutting plans and post-processor output are not changed by the renderer. The CAD
 conversion/program-editing views retain their own source/classification colors.
 
+## Festive schemes
+
+**Halloween**, **Christmas** and **4th of July** are built-in schemes chosen like
+any other; OpenNest never switches to them automatically. Their background,
+sheet and accent colors carry the theme, while each 12-color part palette stays
+varied, light enough for black part IDs and clear of the green etch stroke.
+Etch, cut-outline and selection behavior match Classic.
+
 ## Custom scheme files
 
 Scheme JSON files in `Schemes/` beside the application may specify `etchColor`