Add a resource for title bar background (#21440)

src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml

@@ -825,6 +825,7 @@
       <SolidColorBrush x:Key="CaptionButtonForeground" Color="Black" />
       <SolidColorBrush x:Key="CaptionButtonBackground" Color="#ffe5e5e5" />
       <SolidColorBrush x:Key="CaptionButtonBorderBrush" Color="#ffcacaca" />
+      <StaticResource x:Key="TitleBarBackgroundBrush" ResourceKey="SystemControlBackgroundAltHighBrush" />
     </ResourceDictionary>
     <ResourceDictionary x:Key="Dark">
       <!-- Resources for Button.xaml -->
@@ -1652,6 +1653,7 @@
       <SolidColorBrush x:Key="CaptionButtonForeground" Color="White" />
       <SolidColorBrush x:Key="CaptionButtonBackground" Color="#ffe5e5e5" />
       <SolidColorBrush x:Key="CaptionButtonBorderBrush" Color="#ffcacaca" />
+      <StaticResource x:Key="TitleBarBackgroundBrush" ResourceKey="SystemControlBackgroundAltHighBrush" />
     </ResourceDictionary>
   </ResourceDictionary.ThemeDictionaries>
 </ResourceDictionary>

src/Avalonia.Themes.Fluent/Controls/WindowDrawnDecorations.xaml

@@ -48,7 +48,7 @@
               <!-- Titlebar: background, title text, and drag area live in underlay -->
               <Panel x:Name="PART_TitleBar" VerticalAlignment="Top"
                      Height="{TemplateBinding TitleBarHeight}"
-                     Background="{DynamicResource SystemControlTransparentBrush}"
+                     Background="{DynamicResource TitleBarBackgroundBrush}"
                      IsVisible="{TemplateBinding HasTitleBar}"
                      WindowDecorationProperties.ElementRole="TitleBar" />
 

src/Avalonia.Themes.Simple/Accents/Base.xaml

@@ -46,6 +46,7 @@
       <SolidColorBrush x:Key="CaptionButtonForeground" Color="Black" />
       <SolidColorBrush x:Key="CaptionButtonBackground" Color="#ffe5e5e5" />
       <SolidColorBrush x:Key="CaptionButtonBorderBrush" Color="#ffcacaca" />
+      <StaticResource x:Key="TitleBarBackgroundBrush" ResourceKey="ThemeBackgroundBrush" />
       
     </ResourceDictionary>
     <ResourceDictionary x:Key="Dark">
@@ -91,6 +92,7 @@
       <SolidColorBrush x:Key="CaptionButtonForeground" Color="White" />
       <SolidColorBrush x:Key="CaptionButtonBackground" Color="#ffe5e5e5" />
       <SolidColorBrush x:Key="CaptionButtonBorderBrush" Color="#ffcacaca" />
+      <StaticResource x:Key="TitleBarBackgroundBrush" ResourceKey="ThemeBackgroundBrush" />
     </ResourceDictionary>
   </ResourceDictionary.ThemeDictionaries>
   <FontFamily x:Key="ContentControlThemeFontFamily">fonts:Inter#Inter, $Default</FontFamily>

src/Avalonia.Themes.Simple/Controls/WindowDrawnDecorations.xaml

@@ -48,7 +48,7 @@
               <!-- Titlebar: background, title text, and drag area live in underlay -->
               <Panel x:Name="PART_TitleBar" VerticalAlignment="Top"
                      Height="{TemplateBinding TitleBarHeight}"
-                     Background="Transparent"
+                     Background="{DynamicResource TitleBarBackgroundBrush}"
                      IsVisible="{TemplateBinding HasTitleBar}"
                      WindowDecorationProperties.ElementRole="TitleBar" />
             </Panel>