MainWindow.xaml

<dx:ThemedWindow x:Class="AppearanceCustomization.MainWindow"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                 xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
                 xmlns:dxa="http://schemas.devexpress.com/winfx/2008/xaml/accordion"
                 xmlns:dxat="http://schemas.devexpress.com/winfx/2008/xaml/accordion/themekeys"
                 xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"                         
                 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                 dx:ThemeManager.ThemeName="Office2019Colorful"
                 xmlns:local="clr-namespace:AppearanceCustomization"
                 mc:Ignorable="d"
                 Title="Appearance Customization" 
                 DataContext="{dxmvvm:ViewModelSource Type=local:MainViewModel}"
                 Height="450" Width="800">
    <dx:ThemedWindow.Resources>
        <Style TargetType="dxa:AccordionControl">
            <Setter Property="Background" Value="#fdfdfd"/>
            <Setter Property="BorderBrush" Value="#51555f"/>
            <Setter Property="BorderThickness" Value="1"/>
        </Style>
    </dx:ThemedWindow.Resources>
    <Grid Margin="2">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="{DXBinding '@e(accordion).IsExpanded ? 200 : `Auto`'}"/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <dxa:AccordionControl x:Name="accordion" 
                              ItemsSource="{Binding Items}" 
                              SelectedItem="{Binding SelectedItem}"
                              ChildrenPath="Nodes" DisplayMemberPath="Header"
                              IsCollapseButtonVisible="True"
                              HeaderContent="Header"
                              AutoExpandAllItems="True"
                              ExpandItemOnHeaderClick="False"
                              CollapsedItemClickAction="Select" Margin="0,0,10,0">
            <dxa:AccordionControl.HeaderStyle>
                <Style TargetType="dxa:HeaderPresenter" >
                    <Setter Property="Background" Value="#e6f2fa"/>
                    <Setter Property="Foreground" Value="Brown"/>
                    <Setter Property="BorderBrush" Value="#51555f"/>
                    <Setter Property="BorderThickness" Value="0,0,0,1"/>
                </Style>
            </dxa:AccordionControl.HeaderStyle>
            <dxa:AccordionControl.ItemContainerStyle>
                <Style TargetType="dxa:AccordionItem">
                    <Setter Property="Foreground" Value="Red"/>
                    <Setter Property="ShowInCollapsedMode" Value="{Binding ShowInCollapsedMode}"/>
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding IsCustomView}" Value="True">
                            <Setter Property="Background" Value="#e6f2fa" />
                            <Setter Property="BorderBrush" Value="#0072c6"/>
                            <Setter Property="BorderThickness" Value="1"/>
                            <Setter Property="Foreground" Value="Orange"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </dxa:AccordionControl.ItemContainerStyle>
        </dxa:AccordionControl>
        <Grid Grid.Column="1" VerticalAlignment="Top">
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition/>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <TextBlock Margin="0,0,0,10" HorizontalAlignment="Left" VerticalAlignment="Bottom" 
                   Text="{DXBinding '`SelectedItem: ` + SelectedItem.Header'}"/>
            <Button Grid.Row="1" Height="30" Content="Update custom items" Command="{Binding UpdateCustomItemsCommand}"/>
            <ToggleButton Grid.Row="1" Grid.Column="1" Height="30" Margin="10,0,0,0" Content="{DXBinding '@s.IsChecked ? `Expand accordion` : `Collapse accordion`'}" 
                          IsChecked="{DXBinding '@e(accordion).IsExpanded', Converter={dxmvvm:BooleanNegationConverter}}"/>
        </Grid>
    </Grid>
</dx:ThemedWindow>