Rename RegionBrush to SystemRegionColor and use it in default templates

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

@@ -29,7 +29,7 @@
       <Color x:Key="SystemListLowColor">#19000000</Color>
       <Color x:Key="SystemListMediumColor">#33000000</Color>
       <Color x:Key="SystemErrorTextColor">#C50500</Color>
-      <Color x:Key="RegionColor">#FFFFFFFF</Color>
+      <Color x:Key="SystemRegionColor">#FFFFFFFF</Color>
       <Color x:Key="SystemRevealListLowColor">#17000000</Color>
       <Color x:Key="SystemRevealListMediumColor">#2E000000</Color>
     </ResourceDictionary>
@@ -60,7 +60,7 @@
       <Color x:Key="SystemListLowColor">#19FFFFFF</Color>
       <Color x:Key="SystemListMediumColor">#33FFFFFF</Color>
       <Color x:Key="SystemErrorTextColor">#FFF000</Color>
-      <Color x:Key="RegionColor">#FF000000</Color>
+      <Color x:Key="SystemRegionColor">#FF000000</Color>
       <Color x:Key="SystemRevealListLowColor">#18FFFFFF</Color>
       <Color x:Key="SystemRevealListMediumColor">#30FFFFFF</Color>
     </ResourceDictionary>

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

@@ -212,7 +212,7 @@
       <SolidColorBrush x:Key="SystemControlTransparentRevealBackgroundBrush" Color="Transparent" />
       <SolidColorBrush x:Key="SystemControlTransparentRevealBorderBrush" Color="Transparent" />
 
-      <SolidColorBrush x:Key="RegionBrush" Color="{StaticResource RegionColor}" />
+      <SolidColorBrush x:Key="SystemRegionBrush" Color="{StaticResource SystemRegionColor}" />
     </ResourceDictionary>
     <ResourceDictionary x:Key="Dark">
       <SolidColorBrush x:Key="SystemControlBackgroundAccentBrush" Color="{DynamicResource SystemAccentColor}" />
@@ -372,7 +372,7 @@
       <SolidColorBrush x:Key="SystemControlTransparentRevealBackgroundBrush" Color="Transparent" />
       <SolidColorBrush x:Key="SystemControlTransparentRevealBorderBrush" Color="Transparent" />
 
-      <SolidColorBrush x:Key="RegionBrush" Color="{StaticResource RegionColor}" />
+      <SolidColorBrush x:Key="SystemRegionBrush" Color="{StaticResource SystemRegionColor}" />
     </ResourceDictionary>
   </ResourceDictionary.ThemeDictionaries>
 </ResourceDictionary>

src/Avalonia.Themes.Fluent/ColorPaletteResources.Properties.cs

@@ -150,4 +150,9 @@ public partial class ColorPaletteResources
     /// Gets or sets the ListMedium color value.
     /// </summary>
     public Color ListMedium { get => GetColor("SystemListMediumColor"); set => SetColor("SystemListMediumColor", value); }
+    
+    /// <summary>
+    /// Gets or sets the RegionColor color value.
+    /// </summary>
+    public Color RegionColor { get => GetColor("SystemRegionColor"); set => SetColor("SystemRegionColor", value); }
 }

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

@@ -2,7 +2,7 @@
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <ControlTheme x:Key="{x:Type EmbeddableControlRoot}" TargetType="EmbeddableControlRoot">
     <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
-    <Setter Property="Background" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
+    <Setter Property="Background" Value="{DynamicResource SystemRegionBrush}"/>
     <Setter Property="TopLevel.SystemBarColor" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
     <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}"/>
     <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />

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

@@ -1,7 +1,7 @@
 <ResourceDictionary xmlns="https://github.com/avaloniaui"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <ControlTheme x:Key="{x:Type Window}" TargetType="Window">
-    <Setter Property="Background" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
+    <Setter Property="Background" Value="{DynamicResource SystemRegionBrush}"/>
     <Setter Property="TransparencyBackgroundFallback" Value="{DynamicResource SystemControlBackgroundAltHighBrush}" />
     <Setter Property="TopLevel.SystemBarColor" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>
     <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>