Introduce Generic.xaml with default TreeDataGrid styles, and make Fluent.axaml inherit it

Author: maxkatz6

src/Avalonia.Controls.TreeDataGrid/Themes/Fluent.axaml

@@ -1,21 +1,33 @@
 <Styles xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 
+  <StyleInclude Source="/Themes/Generic.axaml" />
+  
   <Styles.Resources>
     <ResourceDictionary>
-      <ResourceDictionary.MergedDictionaries>
-        <MergeResourceInclude Source="/Themes/FluentControls.axaml" />
-      </ResourceDictionary.MergedDictionaries>
+      <ResourceDictionary.ThemeDictionaries>
+        <ResourceDictionary x:Key="Default">
+          <SolidColorBrush x:Key="TreeDataGridGridLinesBrush" Color="{StaticResource SystemListLowColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBackgroundPointerOverBrush" Color="{StaticResource SystemBaseHighColor}" Opacity="0.1" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBackgroundPressedBrush" Color="{StaticResource SystemBaseMediumLowColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBorderBrushPointerOverBrush" Color="Transparent" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBorderBrushPressedBrush" Color="Transparent" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderForegroundPointerOverBrush" Color="{StaticResource SystemBaseHighColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderForegroundPressedBrush" Color="{StaticResource SystemBaseHighColor}" />
+          <SolidColorBrush x:Key="TreeDataGridSelectedCellBackgroundBrush" Color="{StaticResource SystemAccentColor}" Opacity="0.4" />
+        </ResourceDictionary>
+        <ResourceDictionary x:Key="Dark">
+          <SolidColorBrush x:Key="TreeDataGridGridLinesBrush" Color="{StaticResource SystemListLowColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBackgroundPointerOverBrush" Color="{StaticResource SystemBaseHighColor}" Opacity="0.1" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBackgroundPressedBrush" Color="{StaticResource SystemBaseMediumLowColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBorderBrushPointerOverBrush" Color="Transparent" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderBorderBrushPressedBrush" Color="Transparent" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderForegroundPointerOverBrush" Color="{StaticResource SystemBaseHighColor}" />
+          <SolidColorBrush x:Key="TreeDataGridHeaderForegroundPressedBrush" Color="{StaticResource SystemBaseHighColor}" />
+          <SolidColorBrush x:Key="TreeDataGridSelectedCellBackgroundBrush" Color="{StaticResource SystemAccentColor}" Opacity="0.4" />
+        </ResourceDictionary>
+      </ResourceDictionary.ThemeDictionaries>
     </ResourceDictionary>
   </Styles.Resources>
 
-  <Style Selector=":is(TreeDataGridCell)">
-    <Setter Property="Background" Value="Transparent"/>
-    <Setter Property="MinHeight" Value="25"/>
-  </Style>
-
-  <Style Selector=":is(TreeDataGridCell):selected">
-    <Setter Property="Background" Value="{DynamicResource TreeDataGridSelectedCellBackgroundBrush}"/>
-  </Style>
-
 </Styles>