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

Code before:
  /// Overrides the default text style used for the row of weekday
  /// labels at the top of the date picker grid.
  final TextStyle? weekdayStyle;

  /// Overrides the default text style used for each individual day
  /// label in the grid of the date picker.
  ///
  /// The [TextStyle.color] of the [dayStyle] is not used,
  /// [dayForegroundColor] is used instead.
  final TextStyle? dayStyle;

  /// Overrides the default color used to paint the day labels in the
  /// grid of the date picker.
  ///
  /// This will be used instead of the color provided in [dayStyle].
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button, such as `WidgetState.selected`, `WidgetState.hovered`,
  /// `WidgetState.focused`, and `WidgetState.disabled`.
  ///
  /// ```dart
  /// dayBackgroundColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.selected)) {
  ///     return Theme.of(context).colorScheme.primary;
  ///   }
  ///   return Colors.transparent;
  /// })
  /// ```
  ///
  /// See also:
  ///   * [dayOverlayColor] which applies an overlay over the day labels depending on the [WidgetState].
  final WidgetStateProperty<Color?>? dayForegroundColor;

  /// Overrides the default color used to paint the background of the
  /// day labels in the grid of the date picker.
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button, such as `WidgetState.selected`, `WidgetState.hovered`,
  /// `WidgetState.focused`, and `WidgetState.disabled`.
  ///
  /// ```dart
  /// dayBackgroundColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.selected)) {
  ///     return Theme.of(context).colorScheme.primary;
  ///   }
  ///   return Colors.transparent;
  /// })
  /// ```
  /// See also:
  ///   * [dayOverlayColor] which applies an overlay over the day labels depending on the [WidgetState].
  final WidgetStateProperty<Color?>? dayBackgroundColor;

Review comment (suggestion):
We can set this else part to null to remind developers that setting to null will just return the default color:)
```suggestion
  ///   return null; // Use the default color.
```

Code after:
  /// Overrides the default text style used for the row of weekday
  /// labels at the top of the date picker grid.
  final TextStyle? weekdayStyle;

  /// Overrides the default text style used for each individual day
  /// label in the grid of the date picker.
  ///
  /// The [TextStyle.color] of the [dayStyle] is not used,
  /// [dayForegroundColor] is used instead.
  final TextStyle? dayStyle;

  /// Overrides the default color used to paint the day labels in the
  /// grid of the date picker.
  ///
  /// This will be used instead of the color provided in [dayStyle].
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button, such as `WidgetState.selected`, `WidgetState.hovered`,
  /// `WidgetState.focused`, and `WidgetState.disabled`.
  ///
  /// ```dart
  /// dayBackgroundColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.selected)) {
  ///     return Theme.of(context).colorScheme.primary;
  ///   }
  ///   return null; // Use the default color.
  /// })
  /// ```
  ///
  /// See also:
  ///   * [dayOverlayColor] which applies an overlay over the day labels depending on the [WidgetState].
  final WidgetStateProperty<Color?>? dayForegroundColor;

  /// Overrides the default color used to paint the background of the
  /// day labels in the grid of the date picker.
  ///
  /// This supports different colors based on the [WidgetState]s of
  /// the day button, such as `WidgetState.selected`, `WidgetState.hovered`,
  /// `WidgetState.focused`, and `WidgetState.disabled`.
  ///
  /// ```dart
  /// dayBackgroundColor: WidgetStateProperty.resolveWith((Set<WidgetState> states) {
  ///   if (states.contains(WidgetState.selected)) {
  ///     return Theme.of(context).colorScheme.primary;
  ///   }
  ///   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;