Avalonia/samples/ControlCatalog/EditableTextBlock.xaml

<Styles xmlns="https://github.com/avaloniaui" 
        xmlns:cont="clr-namespace:ControlCatalog;assembly=ControlCatalog">
    <Style Selector="cont|EditableTextBlock">
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="Padding" Value="0"/>
        <Setter Property="Template">
            <ControlTemplate>
                <Grid>
                    <TextBlock Text="{TemplateBinding Text}" IsVisible="{TemplateBinding !InEditMode}" />
                    <TextBox Text="{TemplateBinding Text}" IsVisible="{TemplateBinding InEditMode}" Name="PART_TextBox" Foreground="Black" />
                </Grid>
            </ControlTemplate>
        </Setter>
    </Style>
</Styles>