Try this overwrite stackpanel on Window tilte bar.
Make WindowStyle="None" ResizeMode="NoResize" AND adjust margin of controls(MAKE TOP NEGATIVE OF STACKPANEL) to overwrite window stackpanel on title bar.
<Window x:Class="WPF_UI.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
WindowStyle="None" WindowStartupLocation="CenterScreen" ResizeMode="NoResize" Width="920" Height="600" >
<Grid>....other stack panels....
**Here i have take stack panel to overwrite on window title bar.**
<StackPanel Margin="0,-566,0,5" Grid.ColumnSpan="3" Height="25" DataContext="{Binding}">
<StackPanel.Background>
<LinearGradientBrush>
<GradientStop Color="White" Offset=".2" />
<GradientStop Color="Gray" Offset=".5" />
<GradientStop Color="Black" Offset="1" />
</LinearGradientBrush>
</StackPanel.Background>
<TextBlock HorizontalAlignment="Left" VerticalAlignment="Center" Margin="1,5,0,0"> APPLICATION NAME
</TextBlock>
<Button Content="Close" Margin="860,-17,0,0" VerticalAlignment="Center" HorizontalAlignment="Left" Name="btnClose" Width="40" >
<Button.Background>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,2">
<GradientStop Color="Black" Offset=".2" />
<GradientStop Color="Gray" Offset=".5" />
<GradientStop Color="White" Offset="1" />
</LinearGradientBrush>
</Button.Background>
</Button>
</StackPanel>
</Grid>