Pourquoi?
Parce que le Foreground est codé en dur #FFBADDE9 dans le modèle par défaut.
Et comment changer de premier plan?
Créer votre propre modèle personnalisé en copiant celui par défaut dans votre balisage XAML et puis changer la valeur Foreground de la propriété. Votre Style ressemblerait alors à ceci:
<Style x:Key="calendarDayButtonStyle" TargetType="{x:Type CalendarDayButton}">
<Setter Property="TextElement.Foreground" Value="Green"/>
<Setter Property="TextElement.FontSize" Value="20"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CalendarDayButton}">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0:0:0.1"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="HighlightBackground" To="0.5" Storyboard.TargetProperty="Opacity"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="HighlightBackground" To="0.5" Storyboard.TargetProperty="Opacity"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="HighlightBackground" To="0" Storyboard.TargetProperty="Opacity"/>
<DoubleAnimation Duration="0" Storyboard.TargetName="NormalText" To=".35" Storyboard.TargetProperty="Opacity"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="Unselected"/>
<VisualState x:Name="Selected">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="SelectedBackground" To=".75" Storyboard.TargetProperty="Opacity"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CalendarButtonFocusStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="CalendarButtonFocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetName="DayButtonFocusVisual" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="CalendarButtonUnfocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetName="DayButtonFocusVisual" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Collapsed</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ActiveStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="Active"/>
<VisualState x:Name="Inactive">
<Storyboard>
<ColorAnimation Duration="0" Storyboard.TargetName="NormalText" To="#FF777777" Storyboard.TargetProperty="(TextElement.Foreground).(SolidColorBrush.Color)"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="DayStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="RegularDay"/>
<VisualState x:Name="Today">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="TodayBackground" To="1" Storyboard.TargetProperty="Opacity"/>
<ColorAnimation Duration="0" Storyboard.TargetName="NormalText" To="#FFFFFFFF" Storyboard.TargetProperty="(TextElement.Foreground).(SolidColorBrush.Color)"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="BlackoutDayStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="NormalDay"/>
<VisualState x:Name="BlackoutDay">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="Blackout" To=".2" Storyboard.TargetProperty="Opacity"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="TodayBackground" Fill="#FFAAAAAA" Opacity="0" RadiusX="1" RadiusY="1"/>
<Rectangle x:Name="SelectedBackground" Fill="#FFBADDE9" Opacity="0" RadiusX="1" RadiusY="1"/>
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}"/>
<Rectangle x:Name="HighlightBackground" Fill="#FFBADDE9" Opacity="0" RadiusX="1" RadiusY="1"/>
<ContentPresenter x:Name="NormalText" TextElement.Foreground="{TemplateBinding Foreground}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="5,1,5,1" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
<Path x:Name="Blackout" Data="M8.1772461,11.029181 L10.433105,11.029181 L11.700684,12.801641 L12.973633,11.029181 L15.191895,11.029181 L12.844727,13.999395 L15.21875,17.060919 L12.962891,17.060919 L11.673828,15.256231 L10.352539,17.060919 L8.1396484,17.060919 L10.519043,14.042364 z" Fill="#FF000000" HorizontalAlignment="Stretch" Margin="3" Opacity="0" RenderTransformOrigin="0.5,0.5" Stretch="Fill" VerticalAlignment="Stretch"/>
<Rectangle x:Name="DayButtonFocusVisual" IsHitTestVisible="false" RadiusX="1" RadiusY="1" Stroke="#FF45D6FA" Visibility="Collapsed"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<!--<Style.Triggers>
<DataTrigger Binding="{Binding Converter={StaticResource HolidayDaysConverter}}" Value="True">
<Setter Property="Background" Value="Red"/>
</DataTrigger>
</Style.Triggers>-->
</Style>
