Allow Inherit to be specified for the CustomColorTheme as well

This allows for using the system theme accent color when using the CustomColorTheme.

src/MainDemo.Wpf/App.xaml

@@ -12,13 +12,17 @@
     <ResourceDictionary>
       <ResourceDictionary.MergedDictionaries>
         <!-- This is the current way to set up your app's initial theme -->
+        <!-- PrimaryColor and SecondaryColor also support the constant string "Inherit" to specify the color should use the system theme accent color -->
         <materialDesign:BundledTheme BaseTheme="Inherit"
                                      ColorAdjustment="{materialDesign:ColorAdjustment}"
                                      PrimaryColor="DeepPurple"
                                      SecondaryColor="Lime" />
 
-        <!-- If you would prefer to use your own colors, there is an option for that as well: -->
-        <!--<materialDesign:CustomColorTheme BaseTheme="Light" PrimaryColor="Aqua" SecondaryColor="DarkGreen" />-->
+        <!--
+        If you would prefer to use your own colors, there is an option for that as well:
+        PrimaryColor and SecondaryColor also support the constant string "Inherit" to specify the color should use the system theme accent color
+        -->
+        <materialDesign:CustomColorTheme BaseTheme="Light" PrimaryColor="Aqua" SecondaryColor="#FF006400" />
 
         <!-- You can also use the built-in theme dictionaries: -->
         <!--

src/MaterialDesignColors.Wpf/SwatchHelper.cs

@@ -5,8 +5,8 @@ namespace MaterialDesignColors;
 
 public static class SwatchHelper
 {
-    public static IEnumerable<ISwatch> Swatches { get; } = new ISwatch[]
-    {
+    public static IEnumerable<ISwatch> Swatches { get; } =
+    [
         new RedSwatch(),
         new PinkSwatch(),
         new PurpleSwatch(),
@@ -26,7 +26,7 @@ public static class SwatchHelper
         new BrownSwatch(),
         new GreySwatch(),
         new BlueGreySwatch(),
-    };
+    ];
 
     public static IDictionary<MaterialDesignColor, Color> Lookup { get; } = Swatches.SelectMany(o => o.Lookup).ToDictionary(o => o.Key, o => o.Value);
 }

src/MaterialDesignThemes.Wpf/BundledTheme.cs

@@ -70,11 +70,11 @@ PrimaryColor is not PrimaryColor primaryColor ||
         Lazy<Color?> accentColor = new(Theme.GetSystemAccentColor);
 
         Color colorPrimary = primaryColor == MaterialDesignColors.PrimaryColor.Inherit
-                              ? (accentColor.Value ?? SwatchHelper.Lookup.First().Value)
+                              ? (accentColor.Value ?? default)
                               : SwatchHelper.Lookup[(MaterialDesignColor)primaryColor];
 
         Color colorSecondary = secondaryColor == MaterialDesignColors.SecondaryColor.Inherit
-                              ? (accentColor.Value ?? SwatchHelper.Lookup.First().Value)
+                              ? (accentColor.Value ?? default)
                               : SwatchHelper.Lookup[(MaterialDesignColor)secondaryColor];
 
         Theme theme = Theme.Create(baseTheme, colorPrimary, colorSecondary);

src/MaterialDesignThemes.Wpf/CustomColorTheme.cs

@@ -1,4 +1,5 @@
-using System.Windows.Media;
+using System.ComponentModel;
+using System.Windows.Media;
 
 namespace MaterialDesignThemes.Wpf;
 
@@ -19,6 +20,7 @@ public BaseTheme? BaseTheme
     }
 
     private Color? _primaryColor;
+    [TypeConverter(typeof(InheritSystemColorTypeConverter))]
     public Color? PrimaryColor
     {
         get => _primaryColor;
@@ -33,6 +35,7 @@ public Color? PrimaryColor
     }
 
     private Color? _secondaryColor;
+    [TypeConverter(typeof(InheritSystemColorTypeConverter))]
     public Color? SecondaryColor
     {
         get => _secondaryColor;

src/MaterialDesignThemes.Wpf/InheritSystemColorTypeConverter.cs

@@ -0,0 +1,49 @@
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+using System.Windows.Media;
+
+namespace MaterialDesignThemes.Wpf;
+
+internal sealed class InheritSystemColorTypeConverter : TypeConverter
+{
+    private const string Inherit = "Inherit";
+
+    private ColorConverter ColorConverter { get; } = new();
+
+    public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
+        => sourceType == typeof(string) ||
+           ColorConverter.CanConvertFrom(context, sourceType) ||
+           base.CanConvertFrom(context, sourceType);
+
+    public override bool CanConvertTo(ITypeDescriptorContext? context, [NotNullWhen(true)] Type? destinationType)
+        => ColorConverter.CanConvertTo(context, destinationType) ||
+           base.CanConvertTo(context, destinationType);
+
+    public override object ConvertFrom(ITypeDescriptorContext? td, System.Globalization.CultureInfo? ci, object? value)
+    {
+        if (value is null)
+        {
+            throw GetConvertFromException(value);
+        }
+
+        string? s = value as string ?? throw new ArgumentNullException(nameof(value));
+
+        if (string.Equals(s, Inherit, StringComparison.OrdinalIgnoreCase))
+        {
+            return Theme.GetSystemAccentColor() ?? default;
+        }
+
+        return ColorConverter.ConvertFrom(td, ci, s);
+    }
+
+    public override object ConvertTo(ITypeDescriptorContext? context, System.Globalization.CultureInfo? culture, object? value, Type destinationType)
+    {
+        if (value is Color color &&
+            color != default &&
+            color == Theme.GetSystemAccentColor())
+        {
+            return Inherit;
+        }
+        return ColorConverter.ConvertTo(context, culture, value, destinationType);
+    }
+}