DateTimePickerStyles.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style TargetType="{x:Type DatePickerTextBox}">
        <Setter Property="FontSize" Value="16"/>
        <Setter Property="FontFamily" Value="Jetbrains Mono"/>
        <Setter Property="VerticalAlignment" Value="Center"/>
        <Setter Property="Background" Value="{DynamicResource SecondaryBackground}"/>
        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextColor}"/>
        <Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
        <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type DatePickerTextBox}">
                    <Grid>
                        <Grid.Resources>
                            <SolidColorBrush x:Key="WatermarkBrush" Color="#FFAAAAAA"/>
                        </Grid.Resources>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualStateGroup.Transitions>
                                    <VisualTransition GeneratedDuration="0"/>
                                    <VisualTransition GeneratedDuration="0:0:0.1" To="MouseOver"/>
                                </VisualStateGroup.Transitions>
                                <VisualState x:Name="Normal"/>
                                <VisualState x:Name="MouseOver">
                                    <Storyboard>
                                        <ColorAnimation Duration="0" To="#FF99C1E2" Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" Storyboard.TargetName="ContentElement"/>
                                        <ColorAnimation Duration="0" To="#FF99C1E2" Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" Storyboard.TargetName="watermark_decorator"/>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                            <VisualStateGroup x:Name="WatermarkStates">
                                <VisualStateGroup.Transitions>
                                    <VisualTransition GeneratedDuration="0"/>
                                </VisualStateGroup.Transitions>
                                <VisualState x:Name="Unwatermarked"/>
                                <VisualState x:Name="Watermarked">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="ContentElement"/>
                                        <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="PART_Watermark"/>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                            <VisualStateGroup x:Name="FocusStates">
                                <VisualStateGroup.Transitions>
                                    <VisualTransition GeneratedDuration="0"/>
                                </VisualStateGroup.Transitions>
                                <VisualState x:Name="Unfocused"/>
                                <VisualState x:Name="Focused">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="FocusVisual"/>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Border x:Name="Border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="1" Opacity="1" Padding="{TemplateBinding Padding}">
                            <Grid x:Name="WatermarkContent" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                                <Border x:Name="ContentElement" BorderBrush="#FFFFFFFF" BorderThickness="1" Visibility="Hidden"/>
                                <Border x:Name="watermark_decorator" BorderBrush="#FFFFFFFF" BorderThickness="0" Visibility="Hidden">
                                    <ContentControl x:Name="PART_Watermark" Focusable="False" IsHitTestVisible="False" Opacity="0" Padding="2"/>
                                </Border>
                                <ScrollViewer x:Name="PART_ContentHost" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="0" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                <Border x:Name="FocusVisual" BorderBrush="#FF45D6FA" CornerRadius="1" IsHitTestVisible="False" Opacity="0"/>
                            </Grid>
                        </Border>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style x:Key="FormDatePicker" TargetType="{x:Type DatePicker}">
        <Setter Property="Background" Value="{DynamicResource SecondaryBackground}"/>
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextColor}"/>
        <Setter Property="FontSize" Value="16"/>
        <Setter Property="FontFamily" Value="Jetbrains Mono"/>
        <Setter Property="Margin" Value="5 0"/>
        <Setter Property="VerticalAlignment" Value="Center"/>
    </Style>
</ResourceDictionary>