Adding support in the PaletteHelper to update custom theme (#2369)

This is saves on adding duplicate resources to the root application.

Author: Keboo

MaterialDesignThemes.Wpf/BundledTheme.cs

@@ -3,7 +3,7 @@
 
 namespace MaterialDesignThemes.Wpf
 {
-    public class BundledTheme : ResourceDictionary
+    public class BundledTheme : ResourceDictionary, IMaterialDesignThemeDictionary
     {
         private BaseTheme? _baseTheme;
         public BaseTheme? BaseTheme

MaterialDesignThemes.Wpf/CustomColorTheme.cs

@@ -3,7 +3,7 @@
 
 namespace MaterialDesignThemes.Wpf
 {
-    public class CustomColorTheme : ResourceDictionary
+    public class CustomColorTheme : ResourceDictionary, IMaterialDesignThemeDictionary
     {
         private BaseTheme? _baseTheme;
         public BaseTheme? BaseTheme

MaterialDesignThemes.Wpf/IMaterialDesignThemeDictionary.cs

@@ -0,0 +1,5 @@
+namespace MaterialDesignThemes.Wpf
+{
+    public interface IMaterialDesignThemeDictionary
+    { }
+}

MaterialDesignThemes.Wpf/PaletteHelper.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Windows;
 
 namespace MaterialDesignThemes.Wpf
@@ -8,23 +9,28 @@ public class PaletteHelper
         public virtual ITheme GetTheme()
         {
             if (Application.Current is null)
-                throw new InvalidOperationException("Cannot get theme outside of a WPF application. Use ResourceDictionaryExtensions.GetTheme on the appropriate resource dictionary instead.");
-            return Application.Current.Resources.GetTheme();
+                throw new InvalidOperationException($"Cannot get theme outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.GetTheme)} on the appropriate resource dictionary instead.");
+            return GetResourceDictionary().GetTheme();
         }
 
         public virtual void SetTheme(ITheme theme)
         {
             if (theme is null) throw new ArgumentNullException(nameof(theme));
             if (Application.Current is null)
-                throw new InvalidOperationException("Cannot set theme outside of a WPF application. Use ResourceDictionaryExtensions.SetTheme on the appropriate resource dictionary instead.");
-            Application.Current.Resources.SetTheme(theme);
+                throw new InvalidOperationException($"Cannot set theme outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.SetTheme)} on the appropriate resource dictionary instead.");
+
+            GetResourceDictionary().SetTheme(theme);
         }
 
         public virtual IThemeManager? GetThemeManager()
         {
             if (Application.Current is null)
-                throw new InvalidOperationException("Cannot get ThemeManager. Use ResourceDictionaryExtensions.GetThemeManager on the appropriate resource dictionary instead.");
-            return Application.Current.Resources.GetThemeManager();
+                throw new InvalidOperationException($"Cannot get ThemeManager outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.GetThemeManager)} on the appropriate resource dictionary instead.");
+            return GetResourceDictionary().GetThemeManager();
         }
+
+        private static ResourceDictionary GetResourceDictionary()
+            => Application.Current.Resources.MergedDictionaries.FirstOrDefault(x => x is IMaterialDesignThemeDictionary) ??
+                Application.Current.Resources;
     }
 }