HandyControl/src/Avalonia/HandyControl_Avalonia/Themes/Styles/Expander.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:hc="clr-namespace:HandyControl.Controls">
    <ControlTemplate x:Key="ExpanderLeftTemplate"
                     TargetType="Expander">
        <Grid ColumnDefinitions="*, Auto">
            <ToggleButton Grid.Column="1"
                          VerticalAlignment="Stretch"
                          VerticalContentAlignment="Stretch"
                          Focusable="False"
                          Padding="0,0,0,10"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          Background="{TemplateBinding Background}"
                          Foreground="{TemplateBinding Foreground}"
                          IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                          MinWidth="{TemplateBinding MinWidth}"
                          Theme="{StaticResource ToggleButtonCustom}">
                <LayoutTransformControl>
                    <LayoutTransformControl.LayoutTransform>
                        <RotateTransform Angle="-90" />
                    </LayoutTransformControl.LayoutTransform>
                    <Grid ColumnDefinitions="*,30">
                        <ContentPresenter Content="{TemplateBinding Header}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path x:Name="PathArrow"
                              IsHitTestVisible="False"
                              Grid.Column="1"
                              Stretch="Uniform"
                              Fill="{TemplateBinding Foreground}"
                              Width="12"
                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                              HorizontalAlignment="Center" />
                    </Grid>
                </LayoutTransformControl>
            </ToggleButton>
            <ContentPresenter Name="PART_ContentPresenter"
                              IsVisible="{TemplateBinding IsExpanded}"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Grid.Column="0" />
        </Grid>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderUpTemplate"
                     TargetType="Expander">
        <Grid RowDefinitions="*,Auto">
            <ToggleButton Grid.Row="1"
                          HorizontalAlignment="Stretch"
                          HorizontalContentAlignment="Stretch"
                          Focusable="False"
                          Padding="10,0,0,0"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          Background="{TemplateBinding Background}"
                          Foreground="{TemplateBinding Foreground}"
                          IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                          MinHeight="{TemplateBinding MinHeight}"
                          Theme="{StaticResource ToggleButtonCustom}">
                <Grid ColumnDefinitions="*,30">
                    <ContentPresenter Content="{TemplateBinding Header}"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                    <Path x:Name="PathArrow"
                          IsHitTestVisible="False"
                          Grid.Column="1"
                          Stretch="Uniform"
                          Fill="{TemplateBinding Foreground}"
                          Width="12"
                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                          HorizontalAlignment="Center" />
                </Grid>
            </ToggleButton>
            <ContentPresenter x:Name="PART_ContentPresenter"
                              IsVisible="{TemplateBinding IsExpanded}"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Grid.Row="0" />
        </Grid>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderRightTemplate"
                     TargetType="Expander">
        <Grid ColumnDefinitions="Auto,*">
            <ToggleButton VerticalAlignment="Stretch"
                          VerticalContentAlignment="Stretch"
                          Focusable="False"
                          Padding="0,0,0,10"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          Background="{TemplateBinding Background}"
                          Foreground="{TemplateBinding Foreground}"
                          IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                          MinWidth="{TemplateBinding MinWidth}"
                          Theme="{StaticResource ToggleButtonCustom}">
                <LayoutTransformControl>
                    <LayoutTransformControl.LayoutTransform>
                        <RotateTransform Angle="-90" />
                    </LayoutTransformControl.LayoutTransform>
                    <Grid ColumnDefinitions="*,30">
                        <ContentPresenter Content="{TemplateBinding Header}"
                                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        <Path x:Name="PathArrow"
                              IsHitTestVisible="False"
                              Grid.Column="1"
                              Stretch="Uniform"
                              Fill="{TemplateBinding Foreground}"
                              Width="12"
                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                              HorizontalAlignment="Center" />
                    </Grid>
                </LayoutTransformControl>
            </ToggleButton>
            <ContentPresenter x:Name="PART_ContentPresenter"
                              IsVisible="{TemplateBinding IsExpanded}"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Grid.Column="1" />
        </Grid>
    </ControlTemplate>

    <ControlTemplate x:Key="ExpanderDownTemplate"
                     TargetType="Expander">
        <Grid RowDefinitions="Auto,*">
            <ToggleButton HorizontalAlignment="Stretch"
                          HorizontalContentAlignment="Stretch"
                          Focusable="False"
                          Padding="10,0,0,0"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          Background="{TemplateBinding Background}"
                          Foreground="{TemplateBinding Foreground}"
                          IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                          MinHeight="{TemplateBinding MinHeight}"
                          Theme="{StaticResource ToggleButtonCustom}">
                <Grid ColumnDefinitions="*,30">
                    <ContentPresenter Content="{TemplateBinding Header}"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                    <Path x:Name="PathArrow"
                          IsHitTestVisible="False"
                          Grid.Column="1"
                          Stretch="Uniform"
                          Fill="{TemplateBinding Foreground}"
                          Width="12"
                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                          HorizontalAlignment="Center" />
                </Grid>
            </ToggleButton>
            <ContentPresenter x:Name="PART_ContentPresenter"
                              IsVisible="{TemplateBinding IsExpanded}"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Grid.Row="1" />
        </Grid>
    </ControlTemplate>

    <ControlTheme x:Key="ExpanderBaseStyle"
                  TargetType="Expander">
        <Setter Property="Foreground"
                Value="{DynamicResource PrimaryTextBrush}" />
        <Setter Property="Background"
                Value="{DynamicResource SecondaryRegionBrush}" />
        <Setter Property="HorizontalContentAlignment"
                Value="Left" />
        <Setter Property="VerticalContentAlignment"
                Value="Center" />
        <Setter Property="BorderBrush"
                Value="{DynamicResource BorderBrush}" />
        <Setter Property="BorderThickness"
                Value="1" />
        <Setter Property="MinHeight"
                Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="MinWidth"
                Value="{StaticResource DefaultControlHeight}" />
        <Setter Property="hc:BorderElement.CornerRadius"
                Value="{StaticResource DefaultCornerRadius}" />

        <Style Selector="^:left">
            <Setter Property="Template"
                    Value="{StaticResource ExpanderLeftTemplate}" />
        </Style>

        <Style Selector="^:up">
            <Setter Property="Template"
                    Value="{StaticResource ExpanderUpTemplate}" />
        </Style>

        <Style Selector="^:right">
            <Setter Property="Template"
                    Value="{StaticResource ExpanderRightTemplate}" />
        </Style>

        <Style Selector="^:down">
            <Setter Property="Template"
                    Value="{StaticResource ExpanderDownTemplate}" />
        </Style>

        <Style Selector="^[IsExpanded=True]">
            <Style Selector="^:left /template/ ToggleButton">
                <Setter Property="hc:BorderElement.CornerRadius"
                        Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='0,1,1,0'}">
                </Setter>
            </Style>
            <Style Selector="^:up /template/ ToggleButton">
                <Setter Property="hc:BorderElement.CornerRadius"
                        Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='0,0,1,1'}">
                </Setter>
            </Style>
            <Style Selector="^:right /template/ ToggleButton">
                <Setter Property="hc:BorderElement.CornerRadius"
                        Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='1,0,0,1'}">
                </Setter>
            </Style>
            <Style Selector="^:down /template/ ToggleButton">
                <Setter Property="hc:BorderElement.CornerRadius"
                        Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='1,1,0,0'}">
                </Setter>
            </Style>
            <Style Selector="^:left /template/ Path#PathArrow, ^:up /template/ Path#PathArrow">
                <Setter Property="Data"
                        Value="{StaticResource DownGeometry}" />
            </Style>
            <Style Selector="^:right /template/ Path#PathArrow, ^:down /template/ Path#PathArrow">
                <Setter Property="Data"
                        Value="{StaticResource UpGeometry}" />
            </Style>
        </Style>

        <Style Selector="^[IsExpanded=False]">
            <Style Selector="^ /template/ ToggleButton">
                <Setter Property="hc:BorderElement.CornerRadius"
                        Value="{Binding Path=(hc:BorderElement.CornerRadius),RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource CornerRadiusSplitConverter}, ConverterParameter='1,1,1,1'}">
                </Setter>
            </Style>
            <Style Selector="^:left /template/ Path#PathArrow, ^:up /template/ Path#PathArrow">
                <Setter Property="Data"
                        Value="{StaticResource UpGeometry}" />
            </Style>
            <Style Selector="^:right /template/ Path#PathArrow, ^:down /template/ Path#PathArrow">
                <Setter Property="Data"
                        Value="{StaticResource DownGeometry}" />
            </Style>
        </Style>

        <Style Selector="^:disabled">
            <Setter Property="Opacity"
                    Value="0.4" />
        </Style>
    </ControlTheme>

    <ControlTheme x:Key="{x:Type Expander}"
                  BasedOn="{StaticResource ExpanderBaseStyle}"
                  TargetType="Expander" />

    <ControlTheme x:Key="Expander.Small"
                  BasedOn="{StaticResource ExpanderBaseStyle}"
                  TargetType="Expander">
        <Setter Property="MinHeight"
                Value="20" />
        <Setter Property="MinWidth"
                Value="20" />
    </ControlTheme>
</ResourceDictionary>