How to change arrow style for <wpftoolkit:DataGrid.GroupStyle>
Please find the solution for the above question
<ControlTemplate x:Key="newToggleButtonControlTemplate" TargetType="{x:Type ToggleButton}">
<Grid>
<Ellipse Fill="{DynamicResource ExpanderArrowFill}"
Stroke="DarkGray"
HorizontalAlignment="Center"
x:Name="circle"
VerticalAlignment="Center"
Width="20" Height="20"/>
<Path
x:Name="Up_Arrow"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Fill="White"
Data="M 0 0 L 5 5 L 10 0 Z" />
<Path
x:Name="Down_Arrow"
Visibility="Collapsed"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Fill="White"
Data="M 0 5 L 5 0 L 10 5 Z" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Fill" Value="{DynamicResource MouseOverBrush}" TargetName="circle"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Fill" Value="{DynamicResource PressedBrush}" TargetName="circle"/>
</Trigger>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Visibility" Value="Visible" TargetName="Down_Arrow"/>
<Setter Property="Visibility" Value="Collapsed" TargetName="Up_Arrow"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
<Style x:Key="newExpanderStyle" TargetType="{x:Type Expander}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Grid Background="#720151">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*" x:Name="ContentRow"/>
</Grid.RowDefinitions>
<Border
x:Name="Border"
Grid.Row="0"
BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="2,2,0,0" >
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<ToggleButton
Template="{DynamicResource newToggleButtonControlTemplate}"
IsChecked="{Binding Path=IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" OverridesDefaultStyle="True" />
<ContentPresenter Grid.Column="1" Margin="4" RecognizesAccessKey="True" ContentSource="Header" />
</Grid>
</Border>
<Border
x:Name="ExpandSite"
Grid.Row="1"
Visibility="Collapsed"
BorderThickness="1,0,1,1"
CornerRadius="0,0,2,2" >
<ContentPresenter
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Focusable="false"/>
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="True">
<Setter Property="Visibility" Value="Visible" TargetName="ExpandSite"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Thanks to all