problem with window while making simple animation

Hi fellows.

I am making a custom control which will be a window. in Xaml my code looks like this:

(tobe easier to read I omitted some controls)

<Window.Resources><!-- Window style --><Style TargetType="Window"><Setter Property="Background" Value="Transparent"/></Style><!-- Border --><Style TargetType="Border"><Setter Property="BorderThickness" Value="0.001"/><Setter Property="BorderBrush" Value="Transparent"/><Style.Triggers><EventTrigger RoutedEvent="Loaded"><BeginStoryboard><Storyboard><DoubleAnimation Storyboard.TargetProperty="Height" 
                                             From="0" To="480" Duration="0:0:0.2"/><DoubleAnimation Storyboard.TargetProperty="Width" 
                                             From="0" To="800" Duration="0:0:0.2"/></Storyboard></BeginStoryboard></EventTrigger></Style.Triggers></Style><!-- Dock Panel --><Style TargetType="DockPanel"><Setter Property="Background" Value="Transparent"/></Style><!-- Titlebar button --><Style TargetType="Button"><Setter Property="Width" Value="25"/><Setter Property="Height" Value="25"/><Setter Property="Background" Value="Transparent"/><Setter Property="BorderThickness" Value="0"/><Style.Triggers><Trigger Property="IsMouseOver" Value="True"><Setter Property="Background" Value="Blue"/></Trigger></Style.Triggers></Style><!-- Title bar Grid--><Style TargetType="Grid" x:Key="Title"><Setter Property="Background" Value="#FF179EDC"/><Setter Property="VerticalAlignment" Value="Top"/><Setter Property="Height" Value="30"/></Style><!-- TextBox stackpanel --><Style TargetType="TextBox"><Setter Property="Background" Value="#FF179EDC"/><Setter Property="Width" Value="190"/><Setter Property="BorderBrush" Value="#FF0F81B4"/><Style.Triggers><Trigger Property="IsFocused" Value="True"><Setter Property="Background" Value="White"/><Setter Property="Width" Value="200"/><Setter Property="BorderThickness" Value="3"/></Trigger><EventTrigger RoutedEvent="UIElement.GotFocus"><BeginStoryboard><Storyboard><DoubleAnimation Storyboard.TargetProperty="Width" 
                                             From="190" By="10" Duration="0:0:0.2" /></Storyboard></BeginStoryboard></EventTrigger><EventTrigger RoutedEvent="UIElement.LostFocus"><BeginStoryboard><Storyboard><DoubleAnimation Storyboard.TargetProperty="Width" 
                                             From="200" By="-10" Duration="0:0:0.2" /></Storyboard></BeginStoryboard></EventTrigger></Style.Triggers></Style></Window.Resources><Border x:Name="WindowBorder"><DockPanel><Grid  Style="{DynamicResource Title}" DockPanel.Dock="Top"><Grid.ColumnDefinitions><ColumnDefinition Width="30"/><ColumnDefinition Width="30"/><ColumnDefinition Width="30"/><ColumnDefinition Width="*"/><ColumnDefinition Width="100"/><ColumnDefinition Width="*"/><ColumnDefinition Width="200"/></Grid.ColumnDefinitions><Button  Content="X" Grid.Column="0"/><Button Content="M" Grid.Column="1"/><Button  Content="-" Grid.Column="2"/><TextBox x:Name="TXTSearch" Grid.Column="7"  /></Grid></DockPanel></Border>			

it works as simple and good I want it but there is a small problem.
can anyone please tell me what is the BLACK background before rendering the form and how can I clear it?