OnlineMonitorSystem/HandyControl/HandyControl_Shared/Themes/Styles/SideMenu.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:hc="clr-namespace:HandyControl.Controls">

    <Style BasedOn="{StaticResource SideMenuBaseStyle}" TargetType="hc:SideMenu"/>

    <Style x:Key="SideMenuAccordion" BasedOn="{StaticResource SideMenuBaseStyle}" TargetType="hc:SideMenu">
        <Setter Property="ExpandMode" Value="Accordion"/>
        <Setter Property="PanelAreaLength" Value="200"/>
        <Setter Property="ItemContainerStyle" Value="{StaticResource SideMenuItemHeaderAccordionBaseStyle}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="hc:SideMenu">
                    <Border Name="BorderRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
                        <StackPanel x:Name="PART_Panel"/>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>