<Window
  x:Class="wpftest2.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"
  mc:Ignorable="d"
  WindowStartupLocation="CenterScreen"
  WindowStyle="SingleBorderWindow"
  Title="MainWindow" Height="600" Width="800">

  <Window.Resources>
    <Style TargetType="{x:Type TextBox}" x:Key="MyTextBox">
      <Setter Property="SelectionBrush" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}" />
      <Setter Property="BorderBrush" Value="#808080" />
      <Setter Property="VerticalAlignment" Value="Center" />
      <Setter Property="Margin" Value="5" />
      <Setter Property="OverridesDefaultStyle" Value="True" />
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type TextBox}">
            <Border
              x:Name="Border"
              BorderBrush="{TemplateBinding BorderBrush}"
              BorderThickness="1">
              <ScrollViewer
                x:Name="PART_ContentHost"
                Focusable="False"
                HorizontalScrollBarVisibility="Hidden"
                VerticalScrollBarVisibility="Auto" />
            </Border>
            <ControlTemplate.Triggers>
              <Trigger Property="IsMouseOver" Value="True">
                <Setter
                  TargetName="Border"
                  Property="BorderBrush"
                  Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectionBrush}" />
              </Trigger>
              <Trigger Property="IsKeyboardFocused" Value="True">
                <Setter
                  TargetName="Border"
                  Property="BorderBrush"
                  Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectionBrush}" />
              </Trigger>
            </ControlTemplate.Triggers>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition />
      <RowDefinition />
      <RowDefinition />
    </Grid.RowDefinitions>
    <TextBox
      Style="{StaticResource MyTextBox}"
      Text="Source template" />
    <TextBox
      Grid.Row="1"
      Style="{StaticResource MyTextBox}"
      SelectionBrush="Red"
      Text="Red selection" />
    <TextBox
      Grid.Row="2"
      Style="{StaticResource MyTextBox}"
      SelectionBrush="Green"
      Text="Green selection" />
  </Grid>
</Window>