Toolkit2/Toolkit.UI.Controls.Avalonia/SettingsExpander/SettingsExpander.axaml

<ResourceDictionary
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="using:Toolkit.UI.Controls.Avalonia"
    xmlns:ui="using:FluentAvalonia.UI.Controls">
    <ControlTheme x:Key="SettingsExpanderExpanderStyle" TargetType="Expander">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="MinWidth" Value="0" />
        <Setter Property="MinHeight" Value="{DynamicResource SettingsExpanderMinHeight}" />
        <Setter Property="BorderBrush" Value="{DynamicResource ExpanderContentBorderBrush}" />
        <Setter Property="BorderThickness" Value="{DynamicResource ExpanderContentDownBorderThickness}" />
        <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
        <Setter Property="Template">
            <ControlTemplate>
                <StackPanel>
                    <controls:SettingsExpanderToggleButton
                        Name="ToggleButton"
                        MinHeight="{TemplateBinding MinHeight}"
                        Content="{TemplateBinding Header}"
                        IsChecked="{TemplateBinding IsExpanded,
                                                    Mode=TwoWay}"
                        IsEnabled="{TemplateBinding IsEnabled}"
                        Theme="{StaticResource SettingsExpanderToggleButtonStyle}" />

                    <Border
                        Name="ExpanderContent"
                        Background="{TemplateBinding Background}"
                        BackgroundSizing="InnerBorderEdge"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        CornerRadius="{TemplateBinding CornerRadius,
                                                       Converter={StaticResource BottomCornerRadiusFilterConverter}}"
                        IsVisible="False">
                        <ContentPresenter
                            Name="PART_ContentPresenter"
                            Margin="{TemplateBinding Padding}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            IsVisible="{TemplateBinding IsExpanded}" />
                    </Border>
                </StackPanel>
            </ControlTemplate>
        </Setter>
        <Style Selector="^:expanded /template/ Border#ExpanderContent">
            <Setter Property="IsVisible" Value="True" />
        </Style>
    </ControlTheme>
    <ControlTheme x:Key="{x:Type ui:SettingsExpander}" TargetType="ui:SettingsExpander">
        <Setter Property="Background" Value="{DynamicResource ExpanderBackground}" />
        <Setter Property="BorderBrush" Value="{DynamicResource ExpanderHeaderBorderBrush}" />
        <Setter Property="BorderThickness" Value="{DynamicResource ExpanderHeaderBorderThickness}" />
        <Setter Property="Padding" Value="{DynamicResource SettingsExpanderPadding}" />
        <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
        <Setter Property="MinHeight" Value="{DynamicResource SettingsExpanderMinHeight}" />
        <Setter Property="ItemsPanel">
            <ItemsPanelTemplate>
                <StackPanel Spacing="1" />
            </ItemsPanelTemplate>
        </Setter>
        <Setter Property="Template">
            <ControlTemplate>
                <Expander
                    Name="Expander"
                    IsExpanded="{TemplateBinding IsExpanded,
                                                 Mode=TwoWay}"
                    Theme="{StaticResource SettingsExpanderExpanderStyle}">
                    <Expander.Header>
                        <ui:SettingsExpanderItem
                            Name="ContentHost"
                            Padding="{DynamicResource SettingsExpanderPadding}"
                            ActionIconSource="{TemplateBinding ActionIconSource}"
                            Background="Transparent"
                            Content="{TemplateBinding Header}"
                            ContentTemplate="{TemplateBinding HeaderTemplate}"
                            Description="{TemplateBinding Description}"
                            Footer="{TemplateBinding Footer}"
                            FooterTemplate="{TemplateBinding FooterTemplate}"
                            IconSource="{TemplateBinding IconSource}"
                            IsClickEnabled="{TemplateBinding IsClickEnabled}" />
                    </Expander.Header>

                    <ItemsPresenter ItemsPanel="{TemplateBinding ItemsPanel}" />
                </Expander>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:empty /template/ ItemsPresenter#ItemsHost">
            <Setter Property="IsVisible" Value="False" />
        </Style>
    </ControlTheme>
</ResourceDictionary>