DataGrid: do not hardcode padding

src/Avalonia.Controls.DataGrid/DataGridTextColumn.cs

@@ -9,6 +9,7 @@ using Avalonia.Media;
 using System;
 using System.ComponentModel;
 using Avalonia.Layout;
+using Avalonia.Markup.Xaml.MarkupExtensions;
 
 namespace Avalonia.Controls
 {
@@ -17,6 +18,7 @@ namespace Avalonia.Controls
     /// </summary>
     public class DataGridTextColumn : DataGridBoundColumn
     {
+        private const string DATAGRID_TextColumnCellTextBlockMarginKey = "DataGridTextColumnCellTextBlockMargin";
 
         private double? _fontSize;
         private FontStyle? _fontStyle;
@@ -186,7 +188,7 @@ namespace Avalonia.Controls
         {
             TextBlock textBlockElement = new TextBlock
             {
-                Margin = new Thickness(4),
+                [!Layoutable.MarginProperty] = new DynamicResourceExtension(DATAGRID_TextColumnCellTextBlockMarginKey),
                 VerticalAlignment = VerticalAlignment.Center
             };
 

src/Avalonia.Controls.DataGrid/Themes/Default.xaml

@@ -1,5 +1,11 @@
-<Styles xmlns="https://github.com/avaloniaui">
+<Styles xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <!--TODO: Validation and Focus-->
+
+  <Styles.Resources>
+    <Thickness x:Key="DataGridTextColumnCellTextBlockMargin">4</Thickness>
+  </Styles.Resources>
+  
   <Style Selector="DataGridCell">
     <Setter Property="Background" Value="Transparent"/>
     <Setter Property="HorizontalContentAlignment" Value="Stretch" />

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

@@ -1,6 +1,8 @@
 <Styles xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Styles.Resources>
+    <Thickness x:Key="DataGridTextColumnCellTextBlockMargin">12,0,12,0</Thickness>
+    
     <SolidColorBrush x:Key="DropLocationIndicatorBackground"
                      Color="#3F4346" />
     <SolidColorBrush x:Key="DisabledVisualElementBackground"