Set grid splitter min width and height to avoid invisible splitters. Make splitter background lighter.

src/Avalonia.Diagnostics/Views/TreePageView.xaml

@@ -2,7 +2,7 @@
              xmlns:vm="clr-namespace:Avalonia.Diagnostics.ViewModels"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="Avalonia.Diagnostics.Views.TreePageView">
-  <Grid ColumnDefinitions="*,4,3*">
+  <Grid ColumnDefinitions="*,Auto,3*">
     <TreeView Name="tree" Items="{Binding Nodes}" SelectedItem="{Binding SelectedNode, Mode=TwoWay}">
       <TreeView.DataTemplates>
         <TreeDataTemplate DataType="vm:TreeNode"
@@ -20,7 +20,7 @@
       </TreeView.Styles>
     </TreeView>
 
-    <GridSplitter Width="4" Grid.Column="1" />
+    <GridSplitter Grid.Column="1" />
     <ContentControl Content="{Binding Details}" Grid.Column="2" />
   </Grid>
 </UserControl>

src/Avalonia.Themes.Default/GridSplitter.xaml

@@ -2,7 +2,9 @@
 
   <Style Selector="GridSplitter">
     <Setter Property="Focusable" Value="True" />
-    <Setter Property="Background" Value="{DynamicResource ThemeBorderLowBrush}" />
+    <Setter Property="MinWidth" Value="6" />
+    <Setter Property="MinHeight" Value="6" />
+    <Setter Property="Background" Value="{DynamicResource ThemeControlMidBrush}" />
     <Setter Property="PreviewContent">
       <Template>
         <Rectangle Fill="{DynamicResource HighlightBrush}" />