Allow changing expander header background through ExpanderAssist (#2164)

* allow changing expander header background through ExpanderAssist

* switched expander header background property to use Brush instead of SolidColorBrush
moved binding to a higher control in stack

Author: ncksol

MaterialDesignThemes.Wpf/ExpanderAssist.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Windows;
 using System.Windows.Controls;
+using System.Windows.Media;
 
 namespace MaterialDesignThemes.Wpf
 {
@@ -41,5 +42,15 @@ public static double GetHeaderFontSize(Expander element)
         public static void SetHeaderFontSize(Expander element, double value)
             => element.SetValue(HeaderFontSizeProperty, value);
         #endregion
+        
+        #region AttachedProperty : HeaderBackgroundProperty
+        public static readonly DependencyProperty HeaderBackgroundProperty
+            = DependencyProperty.RegisterAttached("HeaderBackground", typeof(Brush), typeof(ExpanderAssist));
+
+        public static Brush GetHeaderBackground(Expander element)
+            => (Brush)element.GetValue(HeaderBackgroundProperty);
+        public static void SetHeaderBackground(Expander element, Brush value)
+            => element.SetValue(HeaderBackgroundProperty, value);
+        #endregion
     }
 }

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Expander.xaml

@@ -247,8 +247,9 @@
                                           IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                                           Style="{StaticResource MaterialDesignHorizontalHeaderStyle}"
                                           TextElement.FontSize="{TemplateBinding wpf:ExpanderAssist.HeaderFontSize}"
-                                          Opacity="0.87"
+                                          Opacity="0.87"                                          
                                           Foreground="{TemplateBinding Foreground}"
+                                          Background="{TemplateBinding wpf:ExpanderAssist.HeaderBackground}"
                                           Content="{TemplateBinding Header}"
                                           ContentTemplate="{TemplateBinding HeaderTemplate}"
                                           ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"