Просто пройдите http://msdn.microsoft.com/en-us/magazine/dd882520.aspx. Изменить цвет фона для средства выбора даты по умолчанию в wpf не так просто, но вы можете выполнить следующие шаги
1) Средство выбора даты имеет свойство CalenderStye, поэтому переопределите Calender Default ControlTemplate
2) В шаблоне элемента управления Calender есть CalenderItemStyle, где вы можете переопределить шаблон по умолчанию для CalenderItemStyle.В этом вы можете настроить цвета фона, а также кнопки заголовка.
<Style x:Key="ItemStyle" TargetType="primitives:CalendarItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="primitives:CalendarItem">
<ControlTemplate.Resources>
<DataTemplate x:Key="DayTitleTemplate">
<TextBlock Text="{Binding}"
HorizontalAlignment="Center" />
</DataTemplate>
</ControlTemplate.Resources>
<DockPanel Name="PART_Root"
LastChildFill="True">
<Button x:Name="PART_PreviousButton"
DockPanel.Dock="Left"
Content="<"
Focusable="False" />
<Button x:Name="PART_NextButton"
DockPanel.Dock="Right"
Content=">"
Focusable="False" />
<Button x:Name="PART_HeaderButton"
DockPanel.Dock="Top"
FontWeight="Bold"
Focusable="False" />
<Grid>
<Grid x:Name="PART_MonthView"
Visibility="Visible">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
</Grid>
<Grid x:Name="PART_YearView"
Visibility="Hidden">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
</Grid>
</Grid>
<Rectangle x:Name="PART_DisabledVisual" Opacity="0" Visibility="Collapsed" Fill="#A5FFFFFF"/>
</DockPanel>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="False">
<Setter TargetName="PART_DisabledVisual" Property="Visibility" Value="Visible" />
</Trigger>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type toolkit:Calendar}}, Path=DisplayMode}" Value="Year">
<Setter TargetName="PART_MonthView" Property="Visibility" Value="Hidden" />
<Setter TargetName="PART_YearView" Property="Visibility" Value="Visible" />
</DataTrigger>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type toolkit:Calendar}}, Path=DisplayMode}" Value="Decade">
<Setter TargetName="PART_MonthView" Property="Visibility" Value="Hidden" />
<Setter TargetName="PART_YearView" Property="Visibility" Value="Visible" />
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="pp" TargetType="toolkit:Calendar">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="toolkit:Calendar">
<StackPanel Name="PART_Root"
Orientation="Horizontal">
<primitives:CalendarItem
Name="PART_CalendarItem"
Style="{StaticResource ItemStyle}"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
VerticalAlignment="Center" />
<Border BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Margin="4 4 0 4">
<ScrollViewer
VerticalScrollBarVisibility="Auto"
Height="{Binding ElementName=PART_CalendarItem,
Path=ActualHeight}"
Width="100">
<ItemsControl
ItemsSource=
"{Binding RelativeSource={RelativeSource
AncestorType=toolkit:Calendar},
Path=SelectedDates}" />
</ScrollViewer>
</Border>
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<toolkit:DatePicker CalendarStyle="{StaticResource pp}" Margin="0,0,0,242"></toolkit:DatePicker>