You need to add
TextBlock.TextAlignment="Center"
to get
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Class="Window1"
Title="ToolBar Full Width Sample" Height="300" Width="300"
Background="#FFFFFFFF" TextBlock.TextAlignment="Center">
<Grid>
</Grid>
</Window>