Repository: flutter/flutter
File: packages/flutter/lib/src/material/date_picker_theme.dart (Dart)

Code before:
  ///   return null; // Use the default color.
  /// })
  /// ```
  /// See also:
  ///   * [dayOverlayColor] which applies an overlay over the day labels depending on the [WidgetState].
  final WidgetStateProperty<Color?>? dayBackgroundColor;

  /// Overrides the default highlight color that's typically used to
  /// indicate that a day in the grid is focused, hovered, or pressed.
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button. The overlay color is usually used with an opacity to
  /// create hover, focus, and press effects.
  ///
  /// ```dart
  /// dayOverlayColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.pressed)) {
  ///     return Colors.blue.withOpacity(0.12);
  ///   }
  ///   if (states.contains(WidgetState.hovered)) {
  ///     return Colors.blue.withOpacity(0.08);
  ///   }
  ///   if (states.contains(WidgetState.focused)) {
  ///     return Colors.blue.withOpacity(0.12);
  ///   }
  ///   return Colors.transparent;
  /// })
  /// ```
  final WidgetStateProperty<Color?>? dayOverlayColor;

  /// Overrides the default shape used to paint the shape decoration of the
  /// day labels in the grid of the date picker.
  ///
  /// If the selected day is the current day, the provided shape with the
  /// value of [todayBackgroundColor] is used to paint the shape decoration of
  /// the day label and the value of [todayBorder] and [todayForegroundColor] is
  /// used to paint the border.
  ///
  /// If the selected day is not the current day, the provided shape with the
  /// value of [dayBackgroundColor] is used to paint the shape decoration of
  /// the day label.
  ///
  /// {@tool dartpad}
  /// This sample demonstrates how to customize the day selector shape decoration
  /// using the [dayShape], [todayForegroundColor], [todayBackgroundColor], and
  /// [todayBorder] properties.
  ///
  /// ** See code in examples/api/lib/material/date_picker/date_picker_theme_day_shape.0.dart **
  /// {@end-tool}
  final WidgetStateProperty<OutlinedBorder?>? dayShape;


Review comment (suggestion):
Would be good to do the same update here:) `Colors.transparent` is also fine. I just feel providing a `null` example contains more info.

Code after:
  ///   return null; // Use the default color.
  /// })
  /// ```
  /// See also:
  ///   * [dayOverlayColor] which applies an overlay over the day labels depending on the [WidgetState].
  final WidgetStateProperty<Color?>? dayBackgroundColor;

  /// Overrides the default highlight color that's typically used to
  /// indicate that a day in the grid is focused, hovered, or pressed.
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button. The overlay color is usually used with an opacity to
  /// create hover, focus, and press effects.
  ///
  /// ```dart
  /// dayOverlayColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.pressed)) {
  ///     return Colors.blue.withOpacity(0.12);
  ///   }
  ///   if (states.contains(WidgetState.hovered)) {
  ///     return Colors.blue.withOpacity(0.08);
  ///   }
  ///   if (states.contains(WidgetState.focused)) {
  ///     return Colors.blue.withOpacity(0.12);
  ///   }
  ///   return null; // Use the default color.
  /// })
  /// ```
  final WidgetStateProperty<Color?>? dayOverlayColor;

  /// Overrides the default shape used to paint the shape decoration of the
  /// day labels in the grid of the date picker.
  ///
  /// If the selected day is the current day, the provided shape with the
  /// value of [todayBackgroundColor] is used to paint the shape decoration of
  /// the day label and the value of [todayBorder] and [todayForegroundColor] is
  /// used to paint the border.
  ///
  /// If the selected day is not the current day, the provided shape with the
  /// value of [dayBackgroundColor] is used to paint the shape decoration of
  /// the day label.
  ///
  /// {@tool dartpad}
  /// This sample demonstrates how to customize the day selector shape decoration
  /// using the [dayShape], [todayForegroundColor], [todayBackgroundColor], and
  /// [todayBorder] properties.
  ///
  /// ** See code in examples/api/lib/material/date_picker/date_picker_theme_day_shape.0.dart **
  /// {@end-tool}
  final WidgetStateProperty<OutlinedBorder?>? dayShape;