Update datepicker placeholder style

Author: harshit7962

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

@@ -47,16 +47,32 @@
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type DatePickerTextBox}">
                     <Grid>
-                        <Border>
-                            <ContentControl x:Name="PART_Watermark"
-                                Opacity="0"
-                                Focusable="False"
-                                IsHitTestVisible="False"
-                                Padding="2"/>
-                        </Border>
+                        <VisualStateManager.VisualStateGroups>
+                            <VisualStateGroup Name="WatermarkStates">
+                                <VisualStateGroup.Transitions>
+                                    <VisualTransition GeneratedDuration="0" />
+                                </VisualStateGroup.Transitions>
+                                <VisualState Name="Unwatermarked" />
+                                <VisualState Name="Watermarked">
+                                    <Storyboard>
+                                        <DoubleAnimation Storyboard.TargetName="PART_Watermark" Storyboard.TargetProperty="Opacity" To="1" Duration="0" />
+                                    </Storyboard>
+                                </VisualState>
+                            </VisualStateGroup>
+                        </VisualStateManager.VisualStateGroups>
+                        
                         <Border Margin="{TemplateBinding Margin}">
                             <Decorator x:Name="PART_ContentHost" Margin="{TemplateBinding Padding}" />
                         </Border>
+                        <Border Margin="{TemplateBinding Margin}">
+                            <!-- This is a placeholder for DatePickerTextBox -->
+                            <ContentControl 
+                                x:Name="PART_Watermark"
+                                Opacity="0"
+                                Margin="{TemplateBinding Padding}"
+                                Focusable="False"
+                                IsHitTestVisible="False" />
+                        </Border>
                     </Grid>
                 </ControlTemplate>
             </Setter.Value>