Fix not able to style the calendar item of a calendar

[michaelmairegger]

MaterialDesignInXamlToolkit/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Calendar.xaml

Lines 667 to 679 in 65c80da

	<CalendarItem
	x:Name="PART_CalendarItem"
	Style="{DynamicResource MaterialDesignCalendarItemPortrait}"
	BorderBrush="{TemplateBinding BorderBrush}"
	BorderThickness="{TemplateBinding BorderThickness}"
	Background="{TemplateBinding Background}"
	Foreground="{TemplateBinding Foreground}"
	wpf:CalendarAssist.IsHeaderVisible="{TemplateBinding wpf:CalendarAssist.IsHeaderVisible}"
	wpf:CalendarAssist.Orientation="{TemplateBinding wpf:CalendarAssist.Orientation}"
	wpf:ElevationAssist.Elevation="{TemplateBinding wpf:ElevationAssist.Elevation}"
	wpf:CalendarAssist.HeaderBackground="{TemplateBinding wpf:CalendarAssist.HeaderBackground}"
	wpf:CalendarAssist.HeaderForeground="{TemplateBinding wpf:CalendarAssist.HeaderForeground}"/>
	</ControlTemplate>

Since line 669 is bound to MaterialDesignCalendarItemPortrait it is not possible to overwrite this with the following code:

<Style x:Key="...." BasedOn="{StaticResource MaterialDesignDatePicker}" TargetType="{x:Type DatePicker}"> <Setter Property="CalendarStyle" Value="{StaticResource MyCustomCalendarStyle}" /> </Style> 

The changes in this PR addresses this issue