Semi.Avalonia/demo/Semi.Avalonia.Demo/Themes/ToggleButton.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ControlTheme x:Key="SplitViewToggleButton" TargetType="ToggleButton">
        <Setter Property="Width" Value="32" />
        <Setter Property="Height" Value="32" />
        <Setter Property="Padding" Value="{DynamicResource ButtonDefaultPadding}" />
        <Setter Property="Foreground" Value="{DynamicResource ButtonDefaultTertiaryForeground}" />
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="CornerRadius" Value="{DynamicResource ButtonCornerRadius}" />
        <Setter Property="Cursor" Value="Hand" />
        <Setter Property="Content"
                Value="M5 2H19C20.6569 2 22 3.34315 22 5V19C22 20.6569 20.6569 22 19 22H5C3.34315 22 2 20.6569 2 19V5C2 3.34315 3.34315 2 5 2ZM6 4C5.44772 4 5 4.44772 5 5V19C5 19.5523 5.44772 20 6 20H9C9.55229 20 10 19.5523 10 19V5C10 4.44772 9.55229 4 9 4H6Z" />
        <Setter Property="Template">
            <ControlTemplate TargetType="ToggleButton">
                <Border
                    Name="Background"
                    Padding="{TemplateBinding Padding}"
                    Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}"
                    CornerRadius="{TemplateBinding CornerRadius}">
                    <PathIcon
                        Name="Icon"
                        Theme="{DynamicResource InnerPathIcon}"
                        Data="{TemplateBinding Content}"
                        Foreground="{TemplateBinding Foreground}" />
                </Border>
            </ControlTemplate>
        </Setter>
        <Style Selector="^:pointerover">
            <Setter Property="Background" Value="{DynamicResource ButtonDefaultPointeroverBackground}" />
        </Style>
        <Style Selector="^:pressed">
            <Setter Property="Background" Value="{DynamicResource ButtonDefaultPressedBackground}" />
        </Style>
    </ControlTheme>
</ResourceDictionary>