Shaker/Avalonia/SukiUI/Theme/DatePicker.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:icons="clr-namespace:SukiUI.Content"
                    xmlns:sys="using:System"
                    xmlns:suki="https://github.com/kikipoulet/SukiUI">
    <Design.PreviewWith>
        <Border Width="1000"
                Height="500"
                Padding="20">
            <StackPanel VerticalAlignment="Center" Spacing="20">
                <DatePicker Height="60" />
                <DatePicker CornerRadius="10" />
                <DatePicker IsEnabled="False" />
                <DatePicker>
                    <DataValidationErrors.Error>
                        <sys:Exception>
                            <x:Arguments>
                                <x:String>Error</x:String>
                            </x:Arguments>
                        </sys:Exception>
                    </DataValidationErrors.Error>
                </DatePicker>
            </StackPanel>
        </Border>
    </Design.PreviewWith>

    <x:Double x:Key="DatePickerFlyoutPresenterHighlightHeight">40</x:Double>
    <x:Double x:Key="DatePickerFlyoutPresenterItemHeight">40</x:Double>
    <x:Double x:Key="DatePickerFlyoutPresenterAcceptDismissHostGridHeight">41</x:Double>
    <x:Double x:Key="DatePickerThemeMinWidth">296</x:Double>
    <x:Double x:Key="DatePickerThemeMaxWidth">456</x:Double>
    <Thickness x:Key="DatePickerFlyoutPresenterItemPadding">0,3,0,6</Thickness>
    <Thickness x:Key="DatePickerFlyoutPresenterMonthPadding">9,3,0,6</Thickness>
    <Thickness x:Key="DatePickerHostPadding">0,3,0,6</Thickness>
    <Thickness x:Key="DatePickerHostMonthPadding">9,3,0,6</Thickness>
    <x:Double x:Key="DatePickerSpacerThemeWidth">1</x:Double>

    <ControlTheme x:Key="{x:Type DatePicker}" TargetType="DatePicker">
        <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}" />
        <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}" />
        <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
        <Setter Property="BorderBrush" Value="{DynamicResource ThemeControlHighlightMidBrush}" />
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="Height" Value="38" />
        <Setter Property="HorizontalAlignment" Value="Left" />
        <Setter Property="VerticalAlignment" Value="Center" />
        <Setter Property="Template">
            <ControlTemplate>
                <DataValidationErrors>
                    <Grid Name="LayoutRoot" Margin="0,0,0,0">

                        <Button Name="PART_FlyoutButton" Margin="0"
                                Height="{TemplateBinding Height}"
                                Background="Transparent" 
                                BorderThickness="0,0,0,0"
                                CornerRadius="0" Padding="0"
                                IsEnabled="{TemplateBinding IsEnabled}"
                                TemplatedControl.IsTemplateFocusTarget="True">
                            <suki:GlassCard CornerRadius="{DynamicResource SmallCornerRadius}" Height="{TemplateBinding Height}" 
                                            BorderThickness="{TemplateBinding BorderThickness}" Classes="Discrete" >
                            <Grid Margin="14,0"  ColumnDefinitions="Auto,Auto">
                                <Grid Name="PART_ButtonContentGrid"
                                      Grid.Column="0"
                                      VerticalAlignment="Center"
                                      ColumnDefinitions="78*,Auto,132*,Auto,78*">
                                    <TextBlock Name="PART_DayTextBlock"
                                               Padding="0,0,0,0"
                                               HorizontalAlignment="Center"
                                               VerticalAlignment="Center"
                                               FontFamily="{TemplateBinding FontFamily}"
                                               FontSize="{TemplateBinding FontSize}"
                                               FontWeight="{TemplateBinding FontWeight}"
                                               Foreground="{DynamicResource SukiText}"
                                               Text="day" />
                                    <TextBlock Name="PART_MonthTextBlock"
                                               Padding="5,0,0,0"
                                               FontFamily="{TemplateBinding FontFamily}"
                                               FontSize="{TemplateBinding FontSize}"
                                               FontWeight="{TemplateBinding FontWeight}"
                                               Foreground="{DynamicResource SukiText}"
                                               Text="month"
                                               TextAlignment="Left" />
                                    <TextBlock Name="PART_YearTextBlock"
                                               Margin="8,0,0,0"
                                               Padding="0,0,0,0"
                                               HorizontalAlignment="Center"
                                               FontFamily="{TemplateBinding FontFamily}"
                                               FontSize="{TemplateBinding FontSize}"
                                               FontWeight="{TemplateBinding FontWeight}"
                                               Foreground="{DynamicResource SukiText}"
                                               Text="year" />
                                    <Rectangle x:Name="PART_FirstSpacer"
                                               Grid.Column="1"
                                               Width="0"
                                               HorizontalAlignment="Center"
                                               Fill="{DynamicResource DatePickerSpacerFill}" />
                                    <Rectangle x:Name="PART_SecondSpacer"
                                               Grid.Column="3"
                                               Width="0"
                                               HorizontalAlignment="Center"
                                               Fill="{DynamicResource DatePickerSpacerFill}" />
                                </Grid>
                                <PathIcon Grid.Column="1"
                                          Width="16"
                                          Height="16"
                                          Margin="12,0,0,0"
                                          HorizontalAlignment="Right"
                                          Data="{x:Static icons:Icons.Calendar}"
                                          Foreground="{DynamicResource SukiLowText}" />

                            </Grid>
                                </suki:GlassCard>
                        </Button>

                        <Popup Name="PART_Popup"
                               IsLightDismissEnabled="True"
                               PlacementMode="Bottom"
                               PlacementTarget="{TemplateBinding}"
                               WindowManagerAddShadowHint="False">
                            <DatePickerPresenter Name="PART_PickerPresenter" />
                        </Popup>

                    </Grid>
                </DataValidationErrors>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:disabled /template/ Rectangle">
            <Setter Property="Opacity" Value="{DynamicResource ThemeDisabledOpacity}" />
        </Style>

        <!--  Changes foreground for watermark text when SelectedDate is null  -->
        <Style Selector="^:hasnodate /template/ Button#PART_FlyoutButton TextBlock">
            <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundLowBrush}" />
        </Style>
    </ControlTheme>

    <!--  WinUI: DatePickerFlyoutPresenter  -->
    <ControlTheme x:Key="{x:Type DatePickerPresenter}" TargetType="DatePickerPresenter">
        <Setter Property="Width" Value="396" />
        <Setter Property="MinWidth" Value="296" />
        <Setter Property="MaxHeight" Value="398" />
        <Setter Property="FontWeight" Value="Normal" />
        <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}" />
        <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Name="Background"
                        MaxWidth="350"
                        MaxHeight="260"
                        Margin="0,10"
                        Padding="{DynamicResource DateTimeFlyoutBorderPadding}"
                        BorderBrush="{DynamicResource SukiBorderBrush}"
                        BorderThickness="2"
                        BoxShadow="{DynamicResource SukiPopupShadow}"
                        Classes="Card">
                    <Grid Name="ContentRoot" RowDefinitions="*,Auto">
                        <Grid Name="PART_PickerContainer" Margin="0,-15,0,15">
                            <Grid.OpacityMask>
                                <LinearGradientBrush StartPoint="48%,0%" EndPoint="50%,100%">
                                    <GradientStop Offset="0" />
                                    <GradientStop Offset="0.5" Color="Black" />
                                    <GradientStop Offset="1" />
                                </LinearGradientBrush>
                            </Grid.OpacityMask>
                            <Grid.Styles>
                                <Style Selector="ListBoxItem:selected TextBlock">
                                    <Setter Property="FontWeight" Value="Bold" />
                                </Style>
                                <Style Selector="DateTimePickerPanel &gt; ListBoxItem">
                                    <Setter Property="Background" Value="Transparent" />
                                    <Setter Property="Foreground" Value="{DynamicResource SukiText}" />
                                    <Setter Property="Padding" Value="4,2" />
                                    <Setter Property="Template">
                                        <ControlTemplate>
                                            <Border Margin="0,0,10,0"
                                                    Padding="8,4"
                                                    CornerRadius="6">

                                                <ContentPresenter Name="PART_ContentPresenter"
                                                                  Margin="0,0,0,0"
                                                                  Padding="{TemplateBinding Padding}"
                                                                  VerticalAlignment="Center"
                                                                  HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                                                  VerticalContentAlignment="Center"
                                                                  Background="Transparent"
                                                                  BorderBrush="Transparent"
                                                                  BorderThickness="{TemplateBinding BorderThickness}"
                                                                  Content="{TemplateBinding Content}"
                                                                  ContentTemplate="{TemplateBinding ContentTemplate}"
                                                                  CornerRadius="{TemplateBinding CornerRadius}" />

                                            </Border>
                                        </ControlTemplate>
                                    </Setter>
                                </Style>
                            </Grid.Styles>
                            <!--  Column Definitions set in code, ignore here  -->
                            <Panel Name="PART_MonthHost">
                                <ScrollViewer Margin="5,0"
                                              HorizontalScrollBarVisibility="Disabled"
                                              VerticalScrollBarVisibility="Hidden">
                                    <DateTimePickerPanel Name="PART_MonthSelector"
                                                         ItemHeight="{DynamicResource DatePickerFlyoutPresenterItemHeight}"
                                                         PanelType="Month"
                                                         ShouldLoop="True" />
                                </ScrollViewer>

                            </Panel>
                            <Panel Name="PART_DayHost">
                                <ScrollViewer Margin="5,0"
                                              HorizontalScrollBarVisibility="Disabled"
                                              VerticalScrollBarVisibility="Hidden">
                                    <DateTimePickerPanel Name="PART_DaySelector"
                                                         ItemHeight="{DynamicResource DatePickerFlyoutPresenterItemHeight}"
                                                         PanelType="Day"
                                                         ShouldLoop="True" />
                                </ScrollViewer>
                            </Panel>
                            <Panel Name="PART_YearHost">
                                <ScrollViewer Margin="5,0"
                                              HorizontalScrollBarVisibility="Disabled"
                                              VerticalScrollBarVisibility="Hidden">
                                    <DateTimePickerPanel Name="PART_YearSelector"
                                                         ItemHeight="{DynamicResource DatePickerFlyoutPresenterItemHeight}"
                                                         PanelType="Year"
                                                         ShouldLoop="False" />
                                </ScrollViewer>
                            </Panel>
                            <Border Grid.Column="0"
                                    Grid.ColumnSpan="5"
                                    Height="{DynamicResource DatePickerFlyoutPresenterHighlightHeight}"
                                    VerticalAlignment="Center"
                                    Background="{DynamicResource SukiPrimaryColor5}"
                                    CornerRadius="8"
                                    IsHitTestVisible="False"
                                    ZIndex="-1" />
                            <Rectangle Name="PART_FirstSpacer"
                                       Grid.Column="1"
                                       Width="1"
                                       Margin="0,25"
                                       HorizontalAlignment="Center"
                                       Fill="Transparent" />
                            <Rectangle Name="PART_SecondSpacer"
                                       Grid.Column="3"
                                       Width="1"
                                       Margin="0,25"
                                       HorizontalAlignment="Center"
                                       Fill="Transparent" />
                        </Grid>

                        <StackPanel Name="AcceptDismissGrid"
                                    Grid.Row="1"
                                    Margin="0,0,-10,-10"
                                    HorizontalAlignment="Right"
                                    VerticalAlignment="Bottom"
                                    Orientation="Horizontal">
                            <Button Name="PART_AcceptButton"
                                    HorizontalAlignment="Right"
                                    VerticalAlignment="Bottom"
                                    Classes="Basic">
                                <StackPanel Margin="20,0" Orientation="Horizontal">
                                    <PathIcon Width="12"
                                              Height="12"
                                              Data="{x:Static icons:Icons.Check}"
                                              Foreground="{DynamicResource SukiPrimaryColor}" />
                                    <TextBlock Margin="10,0,0,0"
                                               FontSize="16"
                                               FontWeight="{DynamicResource DefaultDemiBold}"
                                               Foreground="{DynamicResource SukiPrimaryColor}"
                                               Text="Apply" />
                                </StackPanel>
                            </Button>

                        </StackPanel>
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>
    </ControlTheme>
</ResourceDictionary>