<UserControl xmlns="//schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="//schemas.microsoft.com/winfx/2006/xaml"
              Width= "400" Height="400">
     <UserControl.Resources>
         <!-- A ContentControl template that defines your background -->
         <ControlTemplate x:Key="BackgroundTemplate" TargetType="ContentControl">
             <Grid>
                 <Border BorderBrush="Black" BorderThickness="2">
                     <Border.Background>
                         <LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
                             <GradientStop Color="Black" Offset="0.953" />
                             <GradientStop Color="White" Offset="0" />
                         </LinearGradientBrush>
                     </Border.Background>
                 </Border>
                 <Path Data="M 0,0 C 0,620 10,10 560,0" Height="60" VerticalAlignment="Top">
                     <Path.Fill>
                         <LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
                             <GradientStop Color="Black" Offset="0" />
                             <GradientStop Color="White" Offset="0.779" />
                         </LinearGradientBrush>
                     </Path.Fill>
                 </Path>
             </Grid>
         </ControlTemplate>
         <!-- A ContentControl Style that references the background template -->
         <Style x:Key="BackgroundStyle" TargetType="ContentControl">
             <Setter Property="Template" 
Value="{StaticResource BackgroundTemplate}" />
         </Style>
     </UserControl.Resources>

     <!-- Typical usage; 

place the background ContentControl behind your body content -->
     <Grid x:Name="LayoutRoot">