StaticResource alters border of TextBox - wpf

Question

I needed a placeholder for TextBox and found this working code:
<Style x:Key="placeHolder" TargetType="{x:Type TextBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Grid>
<TextBox Text="{Binding Path=Text,
RelativeSource={RelativeSource TemplatedParent},
Mode=TwoWay,
UpdateSourceTrigger=PropertyChanged}"
x:Name="textSource"
Background="Transparent"
Panel.ZIndex="2"
BorderThickness="0,0,0,1"/>
<TextBox Text="{TemplateBinding Tag}" Background="{TemplateBinding Background}" Panel.ZIndex="1">
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Foreground" Value="Transparent"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=Text, Source={x:Reference textSource}}" Value="">
<Setter Property="Foreground" Value="LightGray"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
This is my TextBox:
<TextBox x:Name="isci_tb" HorizontalAlignment="Left" Height="25" Margin="0,0,0,0" TextWrapping="Wrap"
VerticalAlignment="Center" Width="120" BorderThickness="0, 0, 0, 1" TextChanged="isci_tb_TextChanged" Tag="išči" Style="{StaticResource placeHolder}">
<TextBox.Background>
<ImageBrush/>
</TextBox.Background>
</TextBox>
Since I am new to XAML I don't see where or what changes the border of my button. I wish to only have the bottom border but instead all 4 borders are shown.

Answer 1

Set the BorderThickness property of the inner TextBox in your template to 0:
<Style x:Key="placeHolder" TargetType="{x:Type TextBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Grid>
<TextBox Text="{Binding Path=Text,
RelativeSource={RelativeSource TemplatedParent},
Mode=TwoWay,
UpdateSourceTrigger=PropertyChanged}"
x:Name="textSource"
Background="Transparent"
Panel.ZIndex="2"
BorderThickness="0,0,0,1"/>
<TextBox Text="{TemplateBinding Tag}" Background="{TemplateBinding Background}" Panel.ZIndex="1"
BorderThickness="0">
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Foreground" Value="Transparent"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=Text, Source={x:Reference textSource}}" Value="">
<Setter Property="Foreground" Value="LightGray"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>