I added a Border to your ToggleButton to make it easier to click.

ToggleButton:

            <Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton">
                <Setter Property="Focusable" Value="False"/>
                <Setter Property="Width" Value="24"/>
                <Setter Property="Height" Value="24"/>
                <Setter Property="Padding" Value="4"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="ToggleButton">
                            <Grid Width="24" Height="24">
                                <Border Background="White" Opacity="0.001"/>
                                <ContentPresenter/>
                                <Path x:Name="Arrow" Fill="Black" Data="M 5,10 L 15,10 L 10,15 Z"/>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property="IsChecked" Value="True">
                                    <Setter TargetName="Arrow" Property="Data" Value="M 5,5 L 15,10 L 5,15 Z"/>
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>

TreeViewItem:

            <Style TargetType="TreeViewItem">
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="TreeViewItem">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition MinWidth="19" Width="Auto"/>
                                    <ColumnDefinition Width="Auto"/>
                                    <ColumnDefinition Width="*"/>
                                </Grid.ColumnDefinitions>
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="Auto"/>
                                    <RowDefinition/>
                                </Grid.RowDefinitions>

                                <ToggleButton Margin="-1,0,0,0" x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" IsChecked="{Binding Path=IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" ClickMode="Press"/>
                                <Border Name="Bd" Grid.Column="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="True">
                                    <ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" MinWidth="20"/>
                                </Border>
                                <ItemsPresenter x:Name="ItemsHost" Grid.Row="1" 
                    Grid.Column="1" Grid.ColumnSpan="2"/>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property="IsExpanded" Value="false">
                                    <Setter TargetName="ItemsHost" Property="Visibility" Value="Collapsed"/>
                                </Trigger>
                                <Trigger Property="HasItems" Value="false">
                                    <Setter TargetName="Expander" Property="Visibility" Value="Hidden"/>
                                </Trigger>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="HasHeader" Value="false"/>
                                        <Condition Property="Width" Value="Auto"/>
                                    </MultiTrigger.Conditions>
                                    <Setter TargetName="PART_Header" Property="MinWidth" Value="75"/>
                                </MultiTrigger>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="HasHeader" Value="false"/>
                                        <Condition Property="Height" Value="Auto"/>
                                    </MultiTrigger.Conditions>
                                    <Setter TargetName="PART_Header" Property="MinHeight" Value="19"/>
                                </MultiTrigger>
                                <Trigger Property="IsSelected" Value="true">
                                    <Setter TargetName="Bd" Property="Background" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
                                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
                                </Trigger>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="IsSelected" Value="true"/>
                                        <Condition Property="IsSelectionActive" Value="false"/>
                                    </MultiTrigger.Conditions>
                                    <Setter TargetName="Bd" Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
                                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
                                </MultiTrigger>
                                <Trigger Property="IsEnabled" Value="false">
                                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>