<Window x:Class="..........MessageWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       
	MinHeight="140" MinWidth="300" 
        Height="140" Width="400" 
        WindowStartupLocation="CenterOwner" 
        AllowsTransparency="True" 
        Background="Transparent" 
        WindowStyle="None" 
        ShowInTaskbar="False">

    <Border Width="Auto" Height="Auto"    
          BorderBrush="Red"
          BorderThickness="2"
          CornerRadius="30,30,30,30" >
        <Border.Background>
            <LinearGradientBrush >
                <GradientBrush.GradientStops>
                    <GradientStopCollection>
                        <GradientStop Offset="0" Color="#DBF6B0"/>
                        <GradientStop Offset="3" Color="#7DC244"/>
                    </GradientStopCollection>
                </GradientBrush.GradientStops>
            </LinearGradientBrush>
        </Border.Background>

................... заголовок, сообщение, кнопки , etc .....................


    </Border>
</Window>