Adding missing Root Template to DatePicker

Author: harshit7962

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Styles/DatePicker.xaml

@@ -104,7 +104,7 @@
                                 BorderBrush="{TemplateBinding BorderBrush}"
                                 BorderThickness="{TemplateBinding BorderThickness}"
                                 CornerRadius="{TemplateBinding Border.CornerRadius}">
-                                <Grid HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
+                                <Grid x:Name="PART_Root" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                                     <Grid.ColumnDefinitions>
                                         <ColumnDefinition Width="*" />
                                         <ColumnDefinition Width="Auto" />