animation.xaml

<Window x:Class="WPFTestApp.MainWindow"
        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"
        xmlns:local="clr-namespace:WPFTestApp"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
<Grid Name="root">

        <Border BorderThickness="2" BorderBrush="Gray" Height="20" Width="150" >
            <Border.Resources>
                <Style TargetType="Border">
                    <Style.Resources>

                        <Storyboard x:Key="animation" x:Name="animation">
                            <DoubleAnimation
                                Storyboard.TargetProperty="BorderBrush.(GradientBrush.GradientStops)[1].(GradientStop.Offset)"
                                From="-.3" To="1.1" Duration="0:0:1"
                                 RepeatBehavior="Forever"
                                />


                        </Storyboard>
                    </Style.Resources>

                    <Setter Property="Background">
                        <Setter.Value>
                            <LinearGradientBrush>
                                <GradientStop Color="Black" Offset="-.4" />
                                <GradientStop Color="White" Offset="-.3" />
                                <!--<GradientStop Color="Black" Offset="-.2" />
                                <GradientStop Color="White" Offset="-.1" />
                                <GradientStop Color="Black" Offset="0" />
                                <GradientStop Color="White" Offset=".1" />
                                <GradientStop Color="Black" Offset=".2" />
                                <GradientStop Color="White" Offset=".3" />
                                <GradientStop Color="Black" Offset=".4" />
                                <GradientStop Color="White" Offset=".5" />
                                <GradientStop Color="Black" Offset=".6" />
                                <GradientStop Color="White" Offset=".7" />
                                <GradientStop Color="Black" Offset=".8" />
                                <GradientStop Color="White" Offset=".9" />
                                <GradientStop Color="Black" Offset="1.0" />
                                <GradientStop Color="White" Offset=".9" />-->
                                <GradientStop Color="Black" Offset="1.0" />
                            </LinearGradientBrush>
                        </Setter.Value>
                    </Setter>
                    <Setter Property="Storyboard.Target" Value="{StaticResource animation}" />


                </Style>
            </Border.Resources>
        </Border>
    </Grid>
</Window>