ILSpy/ILSpy/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ILSpy.App"
             xmlns:local="using:ILSpy"
             xmlns:tree="using:ILSpy.AssemblyTree"
             xmlns:search="using:ILSpy.Search"
             xmlns:analyzers="using:ILSpy.Analyzers"
             xmlns:textView="using:ILSpy.TextView"
             xmlns:metaViews="using:ILSpy.Views"
             xmlns:vm="using:ILSpy.ViewModels"
             xmlns:dockTheme="using:Dock.Avalonia.Themes.Simple"
             xmlns:controls="using:ILSpy.Controls"
             RequestedThemeVariant="Default">
             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->

    <Application.Resources>
        <!-- Override ProDataGrid's hierarchical expander to a classic Windows-Explorer-sized 9x9 box,
             plus 4px of left padding (so the ToggleButton itself is 13x9). -->
        <x:Double x:Key="DataGridHierarchicalExpanderSize">13</x:Double>
        <x:Double x:Key="DataGridHierarchicalExpanderGlyphSize">5</x:Double>

        <!-- Suppress the cell focus rectangle — selected rows already get the highlight background. -->
        <SolidColorBrush x:Key="DataGridCellFocusVisualPrimaryBrush" Color="Transparent" />
        <SolidColorBrush x:Key="DataGridCellFocusVisualSecondaryBrush" Color="Transparent" />

        <!-- Suppress the gray pointer-over highlight on rows and cells. -->
        <SolidColorBrush x:Key="DataGridRowHoveredBackgroundBrush" Color="Transparent" />
        <SolidColorBrush x:Key="DataGridCellHoveredBackgroundBrush" Color="Transparent" />
    </Application.Resources>

    <Application.DataTemplates>
        <DataTemplate DataType="tree:AssemblyTreeModel">
            <tree:AssemblyListPane />
        </DataTemplate>
        <DataTemplate DataType="search:SearchPaneModel">
            <search:SearchPane />
        </DataTemplate>
        <DataTemplate DataType="analyzers:AnalyzerTreeViewModel">
            <analyzers:AnalyzerTreeView />
        </DataTemplate>
        <DataTemplate DataType="vm:ContentTabPage">
            <metaViews:ContentTabPageView />
        </DataTemplate>
        <DataTemplate DataType="textView:DecompilerTabPageModel">
            <textView:DecompilerTextView />
        </DataTemplate>
        <DataTemplate DataType="vm:MetadataTablePageModel">
            <metaViews:MetadataTablePage />
        </DataTemplate>
        <local:ViewLocator/>
    </Application.DataTemplates>

    <Application.Styles>
        <SimpleTheme />
        <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Simple.v2.xaml" />
        <StyleInclude Source="avares://AvaloniaEdit/Themes/Simple/AvaloniaEdit.xaml" />
        <dockTheme:DockSimpleTheme />

        <!-- Classic Windows-Explorer +/- expander. Colors are hardcoded because the
             presenter's :checked style forces Background/BorderBrush to Transparent
             through TemplateBinding. The Margin offsets the visible glyph from the
             presenter cell's left edge — the presenter itself overwrites its Padding
             from Level * Indent in code. -->
        <Style Selector="DataGridHierarchicalPresenter /template/ ToggleButton#PART_Expander">
            <Setter Property="Padding" Value="0" />
            <Setter Property="Template">
                <ControlTemplate TargetType="ToggleButton">
                    <Border Width="9" Height="9"
                            Margin="4,0,0,0"
                            BorderThickness="1"
                            BorderBrush="#828790"
                            CornerRadius="1"
                            HorizontalAlignment="Center"
                            VerticalAlignment="Center">
                        <Border.Background>
                            <LinearGradientBrush StartPoint="0%,0%" EndPoint="100%,100%">
                                <GradientStop Color="White" Offset="0.2" />
                                <GradientStop Color="#FFC0B7A6" Offset="1" />
                            </LinearGradientBrush>
                        </Border.Background>
                        <Path Name="ExpandPath"
                              Margin="1"
                              Fill="Black"
                              Data="M 0 2 L 0 3 L 2 3 L 2 5 L 3 5 L 3 3 L 5 3 L 5 2 L 3 2 L 3 0 L 2 0 L 2 2 Z" />
                    </Border>
                </ControlTemplate>
            </Setter>
        </Style>
        <Style Selector="DataGridHierarchicalPresenter /template/ ToggleButton#PART_Expander:checked /template/ Path#ExpandPath">
            <Setter Property="Data" Value="M 0 2 L 0 3 L 5 3 L 5 2 Z" />
        </Style>
        <!-- Hide the +/- on leaves but keep the column width so siblings stay aligned. -->
        <Style Selector="DataGridHierarchicalPresenter /template/ ToggleButton#PART_Expander:disabled">
            <Setter Property="Opacity" Value="0" />
        </Style>

        <!-- Selected row text in white so it reads against the accent-color highlight. -->
        <Style Selector="DataGridRow:selected">
            <Setter Property="Foreground" Value="White" />
        </Style>

        <!-- Override the presenter template so we can layer classic Windows-Explorer tree
             lines behind the indent / expander / content. -->
        <Style Selector="DataGridHierarchicalPresenter">
            <Setter Property="Template">
                <ControlTemplate TargetType="DataGridHierarchicalPresenter">
                    <Panel>
                        <controls:TreeLines Node="{Binding Content, RelativeSource={RelativeSource TemplatedParent}}"
                                            Level="{Binding Level, RelativeSource={RelativeSource TemplatedParent}}" />
                        <Border Padding="{TemplateBinding Padding}">
                            <Grid ColumnDefinitions="Auto,*">
                                <ToggleButton x:Name="PART_Expander"
                                              Width="{DynamicResource DataGridHierarchicalExpanderSize}"
                                              Height="{DynamicResource DataGridHierarchicalExpanderSize}"
                                              Theme="{DynamicResource DataGridHierarchicalExpanderTheme}"
                                              IsEnabled="{TemplateBinding IsExpandable}"
                                              HorizontalAlignment="Center"
                                              VerticalAlignment="Center"
                                              IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" />
                                <ContentPresenter Grid.Column="1"
                                                  Margin="{DynamicResource DataGridHierarchicalPresenterContentMargin}"
                                                  HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                                  VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                                  ContentTemplate="{TemplateBinding ContentTemplate}"
                                                  Content="{TemplateBinding Content}" />
                            </Grid>
                        </Border>
                    </Panel>
                </ControlTemplate>
            </Setter>
        </Style>
    </Application.Styles>
</Application>