Avalonia/samples/XamlTestApplicationPcl/TextBox.paml

<Styles xmlns="https://github.com/perspex"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:XamlTestApplication;assembly=XamlTestApplicationPcl">
  <Style Selector="TextBox">
    <Setter Property="Background" Value="White"/>
    <Setter Property="BorderBrush" Value="#ff707070"/>
    <Setter Property="BorderThickness" Value="2"/>
    <Setter Property="Padding" Value="2"/>
    <Setter Property="Template">
      <ControlTemplate>
        <Border Name="border"
                Background="{TemplateBinding Background}"
                BorderBrush="{TemplateBinding BorderBrush}"
                BorderThickness="{TemplateBinding BorderThickness}"
                Padding="2">
          <ScrollViewer CanScrollHorizontally="{TemplateBinding CanScrollHorizontally}"
                        HorizontalScrollBarVisibility="{TemplateBinding HorizontalScrollBarVisibility}"
                        VerticalScrollBarVisibility="{TemplateBinding VerticalScrollBarVisibility}">
            <StackPanel Margin="{TemplateBinding Padding}">
              <TextBlock Name="floatingWatermark"
                         Foreground="#ff007ACC"
                         FontSize="10"
                         Text="{TemplateBinding Watermark}"
                         IsVisible="{TemplateBinding FloatingWatermark}">
              </TextBlock>
              <Panel>
                <TextBlock Name="watermark"
                           Opacity="0.5"
                           Text="{TemplateBinding Watermark}"
                           IsVisible="{TemplateBinding Path=Text, Converter={Static StringConverters.NullOrEmpty}}"/>
                <TextPresenter Name="PART_TextPresenter"
                               CaretIndex="{TemplateBinding CaretIndex}"
                               SelectionStart="{TemplateBinding SelectionStart}"
                               SelectionEnd="{TemplateBinding SelectionEnd}"
                               Text="{TemplateBinding Text}"
                               TextWrapping="{TemplateBinding TextWrapping}"/>
              </Panel>
            </StackPanel>
          </ScrollViewer>
        </Border>
      </ControlTemplate>
    </Setter>
  </Style>
  <Style Selector="TextBox:focus /template/ Border#border">
    <Setter Property="BorderBrush" Value="Black"/>
  </Style>
</Styles>