Fix ToolBar overflow button background not inheriting from ToolBar

Change the overflow button styles to bind their Background to the
ancestor ToolBar's Background instead of hardcoding the theme resource.
This ensures the overflow button matches when a custom Background is
set on the ToolBar.

Changes:
- Both vertical and horizontal overflow button styles now use a
  RelativeSource binding to the parent ToolBar's Background
- Overflow popup border uses TemplateBinding to inherit Background
- Removed redundant Background reset in the vertical orientation trigger

Fixes #3694

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Keboo

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ToolBar.xaml

@@ -17,7 +17,7 @@
   </ResourceDictionary.MergedDictionaries>
 
   <Style x:Key="MaterialDesignToolBarVerticalOverflowButtonStyle" TargetType="{x:Type ToggleButton}">
-    <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.ToolBar.Background}" />
+    <Setter Property="Background" Value="{Binding Background, RelativeSource={RelativeSource AncestorType=ToolBar}}" />
     <Setter Property="MinHeight" Value="0" />
     <Setter Property="MinWidth" Value="0" />
     <Setter Property="Template">
@@ -55,7 +55,7 @@
   </Style>
 
   <Style x:Key="MaterialDesignToolBarHorizontalOverflowButtonStyle" TargetType="{x:Type ToggleButton}">
-    <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.ToolBar.Background}" />
+    <Setter Property="Background" Value="{Binding Background, RelativeSource={RelativeSource AncestorType=ToolBar}}" />
     <Setter Property="MinHeight" Value="0" />
     <Setter Property="MinWidth" Value="0" />
     <Setter Property="Template">
@@ -187,7 +187,7 @@
                      CacheMode="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:ShadowAssist.CacheMode)}">
                 <Border x:Name="ToolBarSubMenuBorder"
                         Margin="1"
-                        Background="{DynamicResource MaterialDesign.Brush.ToolBar.Background}"
+                        Background="{TemplateBinding Background}"
                         BorderBrush="{DynamicResource MaterialDesign.Brush.ToolBar.Overflow.Border}"
                         BorderThickness="1"
                         CornerRadius="2"
@@ -246,7 +246,6 @@
               <Setter TargetName="ToolBarSubMenuBorder" Property="Margin" Value="5,5,5,5" />
             </Trigger>
             <Trigger Property="Orientation" Value="Vertical">
-              <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.ToolBar.Background}" />
               <Setter TargetName="Grid" Property="Margin" Value="1,3,1,1" />
               <Setter TargetName="MainPanelBorder" Property="Margin" Value="0,0,0,11" />
               <Setter TargetName="OverflowButton" Property="Style" Value="{StaticResource MaterialDesignToolBarVerticalOverflowButtonStyle}" />

tests/MaterialDesignThemes.UITests/WPF/ToolBars/ToolBarTests.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Windows.Media;
 
 
 namespace MaterialDesignThemes.UITests.WPF.ToolBars;
@@ -30,4 +31,28 @@ public async Task ToolBar_OverflowGrid_RespectsOrientation(Orientation orientati
 
         recorder.Success();
     }
+
+    [Description("Issue 3694")]
+    [Test]
+    public async Task ToolBar_OverflowButton_InheritsCustomBackground()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        //Arrange
+        var toolBarTray = await LoadXaml<ToolBarTray>(@"
+<ToolBarTray DockPanel.Dock=""Top"">
+  <ToolBar Style=""{StaticResource MaterialDesignToolBar}"" Background=""Fuchsia"" OverflowMode=""Always"">
+    <Button Content=""{materialDesign:PackIcon Kind=File}""/>
+  </ToolBar>
+</ToolBarTray>");
+        var overflowButton = await toolBarTray.GetElement<ToggleButton>("OverflowButton");
+
+        //Act
+        Color? overflowBackground = await overflowButton.GetBackgroundColor();
+
+        //Assert
+        await Assert.That(overflowBackground).IsEqualTo(Colors.Fuchsia);
+
+        recorder.Success();
+    }
 }