Improve styling

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -215,6 +215,7 @@ namespace Avalonia.Controls.Primitives
                 // Week-number cells — added after existing cells to preserve child indices 0–48.
                 // Header placeholder (row 0, col 0)
                 _weekNumberHeaderLabel = new CalendarWeekNumberLabel();
+                _weekNumberHeaderLabel.IsHeader = true;
                 _weekNumberHeaderLabel.SetValue(Grid.RowProperty, 0);
                 _weekNumberHeaderLabel.SetValue(Grid.ColumnProperty, 0);
                 children.Add(_weekNumberHeaderLabel);

src/Avalonia.Controls/Calendar/CalendarWeekNumberLabel.cs

@@ -4,8 +4,26 @@ namespace Avalonia.Controls.Primitives
     /// Displays a week number in the month view of a <see cref="Calendar"/>.
     /// Apply styles targeting <see cref="CalendarWeekNumberLabel"/> to customise
     /// the appearance — for example <c>FontWeight="Bold"</c>.
+    /// Use the <c>:header</c> pseudo-class to target the column header cell (row 0).
     /// </summary>
     public sealed class CalendarWeekNumberLabel : ContentControl
     {
+        private bool _isHeader;
+
+        /// <summary>
+        /// Gets or sets a value indicating whether this label is the column header cell
+        /// (placed in row 0 of the month grid, above the week-number data cells).
+        /// Themes can target this with the <c>:header</c> pseudo-class.
+        /// </summary>
+        public bool IsHeader
+        {
+            get => _isHeader;
+            internal set
+            {
+                if (_isHeader == value) return;
+                _isHeader = value;
+                PseudoClasses.Set(":header", value);
+            }
+        }
     }
 }

src/Avalonia.Themes.Fluent/Accents/FluentControlResources.xaml

@@ -402,6 +402,8 @@
       <StaticResource x:Key="CalendarViewBorderBrush" ResourceKey="SystemControlForegroundChromeMediumBrush" />
       <StaticResource x:Key="CalendarViewWeekDayForegroundDisabled"
                       ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
+      <StaticResource x:Key="CalendarViewWeekNumberSeparatorForeground" 
+                      ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonBackground" ResourceKey="SystemControlTransparentBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonForegroundPointerOver"
                       ResourceKey="SystemControlForegroundBaseHighBrush" />
@@ -1228,6 +1230,8 @@
       <StaticResource x:Key="CalendarViewBorderBrush" ResourceKey="SystemControlForegroundChromeMediumBrush" />
       <StaticResource x:Key="CalendarViewWeekDayForegroundDisabled"
                       ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
+      <StaticResource x:Key="CalendarViewWeekNumberSeparatorForeground" 
+                      ResourceKey="SystemControlDisabledBaseMediumLowBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonBackground" ResourceKey="SystemControlTransparentBrush" />
       <StaticResource x:Key="CalendarViewNavigationButtonForegroundPointerOver"
                       ResourceKey="SystemControlForegroundBaseHighBrush" />

src/Avalonia.Themes.Fluent/Controls/CalendarWeekNumberLabel.xaml

@@ -5,21 +5,35 @@
   <ControlTheme x:Key="{x:Type CalendarWeekNumberLabel}" TargetType="CalendarWeekNumberLabel">
     <Setter Property="MinWidth" Value="40" />
     <Setter Property="MinHeight" Value="40" />
-    <Setter Property="Margin" Value="1" />
     <Setter Property="Padding" Value="0" />
     <Setter Property="Foreground" Value="{DynamicResource CalendarViewOutOfScopeForeground}" />
     <Setter Property="HorizontalContentAlignment" Value="Center" />
     <Setter Property="VerticalContentAlignment" Value="Center" />
+    <Setter Property="FontSize" Value="{DynamicResource CalendarDayButtonFontSize}"></Setter>
     <Setter Property="Template">
       <ControlTemplate>
-        <ContentPresenter Name="PART_ContentPresenter"
-                          ContentTemplate="{TemplateBinding ContentTemplate}"
-                          Content="{TemplateBinding Content}"
-                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
-                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
-                          Margin="{TemplateBinding Padding}" />
+        <Border BorderBrush="{DynamicResource CalendarViewWeekNumberSeparatorForeground}"
+                BorderThickness="0,0,1,0">
+          <ContentPresenter Name="PART_ContentPresenter"
+                            ContentTemplate="{TemplateBinding ContentTemplate}"
+                            Content="{TemplateBinding Content}"
+                            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                            Margin="{TemplateBinding Padding}" />
+        </Border>
       </ControlTemplate>
     </Setter>
+
+    <!-- Header cell (row 0): match the day-of-week title row appearance -->
+    <Style Selector="^:header">
+      <Setter Property="MinHeight" Value="38" />
+      <Setter Property="MaxHeight" Value="38" />
+      <Setter Property="Padding" Value="0" />
+      <Setter Property="FontStyle" Value="Normal" />
+      <Setter Property="HorizontalContentAlignment" Value="Center" />
+      <Setter Property="Foreground" Value="{DynamicResource CalendarViewForeground}" />
+      <Setter Property="FontWeight" Value="SemiBold" />
+    </Style>
   </ControlTheme>
 
 </ResourceDictionary>

src/Avalonia.Themes.Simple/Controls/CalendarWeekNumberLabel.xaml

@@ -3,21 +3,35 @@
                     x:ClassModifier="internal">
 
   <ControlTheme x:Key="{x:Type CalendarWeekNumberLabel}" TargetType="CalendarWeekNumberLabel">
-    <Setter Property="Margin" Value="0,4,4,4" />
-    <Setter Property="Padding" Value="4,0" />
+    <Setter Property="Margin" Value="0,4,0,4" />
+    <Setter Property="Padding" Value="4,0,6,0" />
+    <Setter Property="FontSize" Value="9.5" />
+    <Setter Property="FontStyle" Value="Italic" />
     <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundLowBrush}" />
     <Setter Property="HorizontalContentAlignment" Value="Center" />
     <Setter Property="VerticalContentAlignment" Value="Center" />
     <Setter Property="Template">
       <ControlTemplate>
-        <ContentPresenter Name="PART_ContentPresenter"
-                          ContentTemplate="{TemplateBinding ContentTemplate}"
-                          Content="{TemplateBinding Content}"
-                          HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
-                          VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
-                          Margin="{TemplateBinding Padding}" />
+        <Border BorderBrush="{DynamicResource ThemeBorderLowBrush}"
+                BorderThickness="0,0,1,0">
+          <ContentPresenter Name="PART_ContentPresenter"
+                            ContentTemplate="{TemplateBinding ContentTemplate}"
+                            Content="{TemplateBinding Content}"
+                            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                            Margin="{TemplateBinding Padding}" />
+        </Border>
       </ControlTemplate>
     </Setter>
+
+    <!-- Header cell (row 0): match the DayTitleTemplate appearance (Bold, normal foreground) -->
+    <Style Selector="^:header">
+      <Setter Property="FontStyle" Value="Normal" />
+      <Setter Property="FontWeight" Value="Bold" />
+      <Setter Property="Padding" Value="4,0" />
+      <Setter Property="HorizontalContentAlignment" Value="Center" />
+      <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}" />
+    </Style>
   </ControlTheme>
 
 </ResourceDictionary>