Set BorderBrush
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<Border Padding="4,4,4,4" BorderThickness="2" Margin="2,2,2,2"
Height="Auto" Width="Auto" CornerRadius="7,7,7,7"
BorderBrush="#FF000080" Background="#726CB167">
<TextBlock Height="Auto" Width="Auto" Text="TextBlock"
TextWrapping="Wrap" Foreground="#FF000000" Margin="2,2,2,2"/>
</Border>
</UserControl>
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