Expander Style        

<Style x:Key="ExpanderViewStyle" TargetType="toolkit:ExpanderView">
            <Setter Property="HorizontalAlignment" Value="Stretch"/>
            <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
            <Setter Property="ItemsPanel">
                <Setter.Value>
                    <ItemsPanelTemplate>
                        <StackPanel/>
                    </ItemsPanelTemplate>
                </Setter.Value>
            </Setter>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="toolkit:ExpanderView">
                        <Grid>
                            <Grid.Resources>
                                <QuadraticEase x:Key="QuadraticEaseOut" EasingMode="EaseOut"/>
                                <QuadraticEase x:Key="QuadraticEaseInOut" EasingMode="EaseInOut"/>
                            </Grid.Resources>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="41"/>
                                <ColumnDefinition Width="*"/>
                            </Grid.ColumnDefinitions>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto"/>
                                <RowDefinition Height="Auto"/>
                                <RowDefinition Height="Auto"/>
                            </Grid.RowDefinitions>
                            <VisualStateManager.VisualStateGroups>
                                <VisualStateGroup x:Name="ExpansionStates">
                                    <VisualStateGroup.Transitions>
                                        <VisualTransition From="Collapsed" GeneratedDuration="0:0:0.15" To="Expanded">
                                            <Storyboard>
                                                <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Height)" Storyboard.TargetName="ItemsCanvas">
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseOut}" KeyTime="0:0:0.00" Value="0"/>
                                                    <EasingDoubleKeyFrame x:Name="CollapsedToExpandedKeyFrame" EasingFunction="{StaticResource QuadraticEaseOut}" KeyTime="0:0:0.15" Value="1"/>
                                                </DoubleAnimationUsingKeyFrames>
                                                <DoubleAnimation Duration="0" To="1.0" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="ItemsCanvas"/>
                                            </Storyboard>
                                        </VisualTransition>
                                        <VisualTransition From="Expanded" GeneratedDuration="0:0:0.15" To="Collapsed">
                                            <Storyboard>
                                                <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Height)" Storyboard.TargetName="ItemsCanvas">
                                                    <EasingDoubleKeyFrame x:Name="ExpandedToCollapsedKeyFrame" EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.00" Value="1"/>
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.15" Value="0"/>
                                                </DoubleAnimationUsingKeyFrames>
                                                <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="ItemsCanvas">
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.00" Value="1.0"/>
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.15" Value="0.0"/>
                                                </DoubleAnimationUsingKeyFrames>
                                                <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(CompositeTransform.TranslateY)" Storyboard.TargetName="ItemsCanvas">
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.00" Value="0.0"/>
                                                    <EasingDoubleKeyFrame EasingFunction="{StaticResource QuadraticEaseInOut}" KeyTime="0:0:0.15" Value="-35"/>
                                                </DoubleAnimationUsingKeyFrames>
                                            </Storyboard>
                                        </VisualTransition>
                                    </VisualStateGroup.Transitions>
                                    <VisualState x:Name="Collapsed">
                                        <Storyboard>
                                            <DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="(FrameworkElement.Height)" Storyboard.TargetName="ItemsCanvas"/>
                                            <DoubleAnimation Duration="0" To="0.0" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="ItemsCanvas"/>
                                        </Storyboard>
                                    </VisualState>
                                    <VisualState x:Name="Expanded">
                                        <Storyboard>
                                            <DoubleAnimation Duration="0" Storyboard.TargetProperty="(FrameworkElement.Height)" Storyboard.TargetName="ItemsCanvas"/>
                                            <DoubleAnimation Duration="0" To="1.0" Storyboard.TargetProperty="(UIElement.Opacity)" Storyboard.TargetName="ItemsCanvas"/>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                                <VisualStateGroup x:Name="ExpandabilityStates">
                                    <VisualState x:Name="Expandable"/>
                                    <VisualState x:Name="NonExpandable">
                                        <Storyboard>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="ExpandableContent">
                                                <DiscreteObjectKeyFrame KeyTime="0:0:0.0" Value="Collapsed"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="Line">
                                                <DiscreteObjectKeyFrame KeyTime="0:0:0.0" Value="Collapsed"/>
                                            </ObjectAnimationUsingKeyFrames>
                                            <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="NonExpandableContent">
                                                <DiscreteObjectKeyFrame KeyTime="0:0:0.0" Value="Visible"/>
                                            </ObjectAnimationUsingKeyFrames>
                                        </Storyboard>
                                    </VisualState>
                                </VisualStateGroup>
                            </VisualStateManager.VisualStateGroups>
                            <ListBoxItem x:Name="ExpandableContent" Grid.ColumnSpan="2" Grid.Column="0" toolkit:TiltEffect.IsTiltEnabled="True" Grid.Row="0" Grid.RowSpan="2">
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="41"/>
                                        <ColumnDefinition Width="*"/>
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="Auto"/>
                                        <RowDefinition Height="Auto"/>
                                        <RowDefinition Height="Auto"/>
                                    </Grid.RowDefinitions>
                                    <ContentControl x:Name="Header" Grid.ColumnSpan="2" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="0" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Grid.Row="0"/>
                                    <ContentControl x:Name="Expander" ContentTemplate="{TemplateBinding ExpanderTemplate}" Content="{TemplateBinding Expander}" Grid.Column="1" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Margin="11,0,0,0" Grid.Row="1"/>
                                    <Grid x:Name="ExpanderPanel" Background="Transparent" Grid.ColumnSpan="2" Grid.Column="0" Grid.Row="0" Grid.RowSpan="2"/>
                                </Grid>
                            </ListBoxItem>
                            <ContentControl x:Name="NonExpandableContent" Grid.ColumnSpan="2" ContentTemplate="{TemplateBinding NonExpandableHeaderTemplate}" Content="{TemplateBinding NonExpandableHeader}" Grid.Column="0" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Grid.Row="0" Grid.RowSpan="2" Visibility="Collapsed"/>
                            <Canvas x:Name="ItemsCanvas" Grid.Column="1" Margin="11,0,0,0" Opacity="0.0" Grid.Row="2">
                                <Canvas.RenderTransform>
                                    <CompositeTransform TranslateY="0.0"/>
                                </Canvas.RenderTransform>
                                <ItemsPresenter x:Name="Presenter"/>
                            </Canvas>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

Use style in control

<toolkit:ExpanderView x:Name="ExpanderToday"  Style="{StaticResource ExpanderViewStyle}">