feat(portable_text): per-subtree config via PortableTextTheme

packages/sanity/flutter_sanity_portable_text/lib/flutter_sanity_portable_text.dart

@@ -10,11 +10,13 @@
 /// - Custom mark definitions
 /// - Custom text mark recognizers
 /// - Shared config across all Portable Text Widgets
+/// - Per-subtree config overrides via [PortableTextTheme] / [PortableTextStyleOverride]
 ///
 library;
 
 export 'model/markdef_descriptor.dart';
 export 'model/text_block.dart';
 export 'ui/portable_text_block.dart';
 export 'ui/portable_text_config.dart';
+export 'ui/portable_text_theme.dart';
 export 'ui/portable_text_widget.dart';

packages/sanity/flutter_sanity_portable_text/lib/model/text_block.dart

@@ -94,6 +94,11 @@ class Span {
       _$SpanFromJson(json);
 }
 
+/// Resolves custom mark deserializers from [PortableTextConfig.shared].
+///
+/// This runs at JSON-parse time with no [BuildContext], so it must read the context-free
+/// [PortableTextConfig.shared] registry rather than a per-subtree config. Mark *styling* is
+/// still resolved per subtree at render time via `PortableTextConfig.of(context)`.
 List<MarkDef> _markDefsFromJson(final List<dynamic> json) {
   final markDefs = PortableTextConfig.shared.markDefs;
 

packages/sanity/flutter_sanity_portable_text/lib/ui/portable_text_block.dart

@@ -18,7 +18,7 @@ class PortableTextBlock extends StatelessWidget {
 
   @override
   Widget build(final BuildContext context) {
-    final config = PortableTextConfig.shared;
+    final config = PortableTextConfig.of(context);
 
     final spans = model.children
         .map((final span) => _buildInlineSpan(span, Theme.of(context), context))
@@ -52,7 +52,7 @@ class PortableTextBlock extends StatelessWidget {
     final ThemeData theme,
     final BuildContext context,
   ) {
-    final config = PortableTextConfig.shared;
+    final config = PortableTextConfig.of(context);
 
     // Step 1: Start with the base style
     final baseStyle = config.baseStyle(context) ??

packages/sanity/flutter_sanity_portable_text/lib/ui/portable_text_config.dart

@@ -27,7 +27,8 @@ typedef BulletRenderer = InlineSpan Function(BuildContext, TextBlockItem);
 /// configuration can be customized to match the visual design of the app. The default
 /// configuration is based on the Material Design guidelines.
 ///
-/// Note that the configuration is shared across all instances of the [PortableText] widget.
+/// A single global default lives in [shared]; individual subtrees can override the rendering
+/// configuration by wrapping them in a [PortableTextTheme] (resolved via [of]).
 final class PortableTextConfig {
   /// The styles used to render the Portable Text content. The keys are the style names used
   /// in the Portable Text content, such as "h1", "h2", "blockquote", etc. The default styles
@@ -63,15 +64,68 @@ final class PortableTextConfig {
   /// The base style used for rendering the Portable Text content. The default value is the bodyMedium style from the theme.
   TextStyle? Function(BuildContext) baseStyle = defaultBaseStyle;
 
-  /// The shared instance of the PortableTextConfig. This instance is used by all [PortableText] widgets
-  /// in the application. You can customize the configuration by calling the [apply] method.
-  static final PortableTextConfig shared = PortableTextConfig._();
+  /// The default/root configuration, used by [PortableText] widgets when no [PortableTextTheme]
+  /// ancestor provides one (see [of]). It is also the context-free registry consulted at
+  /// JSON-parse time for custom mark deserializers (see `_markDefsFromJson`). You can customize
+  /// it by calling the [apply] method.
+  static final PortableTextConfig shared = PortableTextConfig();
 
   /// The bullet renderer used to render the bullet for list items. The default value is a simple bullet renderer
   /// that handles the default bullet types: number, square, and circle.
   BulletRenderer bulletRenderer = defaultBulletRenderer;
 
-  PortableTextConfig._();
+  PortableTextConfig();
+
+  /// Returns the nearest [PortableTextConfig] supplied by a [PortableTextTheme] ancestor,
+  /// falling back to [shared] when none is present. Mirrors `Theme.of`.
+  static PortableTextConfig of(final BuildContext context) =>
+      PortableTextTheme.maybeOf(context) ?? shared;
+
+  /// Returns a copy of this config with the given fields overridden.
+  ///
+  /// The map fields ([styles], [blocks], [blockContainers], [markDefs]) are MERGED — provided
+  /// keys win, the rest are kept — and scalar fields are replaced when provided. This
+  /// intentionally differs from `ThemeData.copyWith` (which replaces whole fields) because
+  /// these maps are additive registries, so the common case is overriding a single key while
+  /// keeping the others.
+  PortableTextConfig copyWith({
+    final Map<String, TextStyleBuilder>? styles,
+    final Map<String, BlockWidgetBuilder>? blocks,
+    final Map<String, BlockContainerBuilder>? blockContainers,
+    final Map<String, MarkDefDescriptor>? markDefs,
+    final double? listIndent,
+    final EdgeInsets? itemPadding,
+    final TextStyle? Function(BuildContext)? baseStyle,
+    final BulletRenderer? bulletRenderer,
+  }) {
+    final config = PortableTextConfig();
+    config.styles
+      ..clear()
+      ..addAll(this.styles);
+    config.blocks
+      ..clear()
+      ..addAll(this.blocks);
+    config.blockContainers
+      ..clear()
+      ..addAll(this.blockContainers);
+    config.markDefs
+      ..clear()
+      ..addAll(this.markDefs);
+    config.listIndent = this.listIndent;
+    config.itemPadding = this.itemPadding;
+    config.baseStyle = this.baseStyle;
+    config.bulletRenderer = this.bulletRenderer;
+
+    if (styles != null) config.styles.addAll(styles);
+    if (blocks != null) config.blocks.addAll(blocks);
+    if (blockContainers != null) config.blockContainers.addAll(blockContainers);
+    if (markDefs != null) config.markDefs.addAll(markDefs);
+    if (listIndent != null) config.listIndent = listIndent;
+    if (itemPadding != null) config.itemPadding = itemPadding;
+    if (baseStyle != null) config.baseStyle = baseStyle;
+    if (bulletRenderer != null) config.bulletRenderer = bulletRenderer;
+    return config;
+  }
 
   /// Applies the custom configuration to the shared instance of the [PortableTextConfig].
   void apply({
@@ -131,7 +185,7 @@ final class PortableTextConfig {
   static const defaultItemPadding = EdgeInsets.only(bottom: 8);
   static BulletRenderer defaultBulletRenderer =
       (final BuildContext context, final TextBlockItem model) {
-    final textStyle = PortableTextConfig.shared.baseStyle(context);
+    final textStyle = PortableTextConfig.of(context).baseStyle(context);
 
     switch (model.listItem) {
       case ListItemType.number:

packages/sanity/flutter_sanity_portable_text/lib/ui/portable_text_theme.dart

@@ -0,0 +1,73 @@
+import 'package:flutter/material.dart';
+
+import '../flutter_sanity_portable_text.dart';
+
+/// An [InheritedWidget] that supplies a [PortableTextConfig] to its subtree.
+///
+/// Mirrors Flutter's `Theme` / `Theme.of`: descendant Portable Text widgets resolve their
+/// config via [PortableTextConfig.of], which returns the nearest [PortableTextTheme]'s
+/// [config] or falls back to [PortableTextConfig.shared]. Wrap a subtree to render it with a
+/// different configuration without mutating the global [PortableTextConfig.shared].
+class PortableTextTheme extends InheritedWidget {
+  /// The configuration applied to descendant Portable Text widgets.
+  final PortableTextConfig config;
+
+  const PortableTextTheme({
+    super.key,
+    required this.config,
+    required super.child,
+  });
+
+  /// The [PortableTextConfig] of the nearest [PortableTextTheme] ancestor, or `null` if none.
+  static PortableTextConfig? maybeOf(final BuildContext context) =>
+      context.dependOnInheritedWidgetOfExactType<PortableTextTheme>()?.config;
+
+  @override
+  bool updateShouldNotify(final PortableTextTheme oldWidget) =>
+      config != oldWidget.config;
+}
+
+/// Restyles descendant Portable Text by transforming the *inherited* text style for each
+/// named style (e.g. `'normal'`, `'h2'`).
+///
+/// Each entry receives the resolved style produced by the ambient config's builder and
+/// returns the adjusted style, so callers express only the delta:
+///
+/// ```dart
+/// PortableTextStyleOverride(
+///   styles: {'normal': (s) => s.copyWith(fontSize: (s.fontSize ?? 16) - 2)},
+///   child: ...,
+/// )
+/// ```
+///
+/// The inherited builder is captured internally, so referencing the same style name does
+/// not recurse. Everything else (blocks, marks, spacing, parsing) is inherited unchanged.
+class PortableTextStyleOverride extends StatelessWidget {
+  /// Per-style transforms applied on top of the inherited resolved style. Keys are style
+  /// names (`'normal'`, `'h2'`, …); a key that the ambient config does not define is ignored.
+  final Map<String, TextStyle Function(TextStyle)> styles;
+
+  /// The subtree whose Portable Text should be restyled.
+  final Widget child;
+
+  const PortableTextStyleOverride({
+    super.key,
+    required this.styles,
+    required this.child,
+  });
+
+  @override
+  Widget build(final BuildContext context) {
+    final base = PortableTextConfig.of(context);
+    final wrapped = <String, TextStyleBuilder>{
+      for (final entry in styles.entries)
+        if (base.styles[entry.key] case final builder?)
+          entry.key: (final ctx, final inherited) =>
+              entry.value(builder(ctx, inherited)),
+    };
+    return PortableTextTheme(
+      config: base.copyWith(styles: wrapped),
+      child: child,
+    );
+  }
+}

packages/sanity/flutter_sanity_portable_text/lib/ui/portable_text_widget.dart

@@ -45,6 +45,6 @@ Widget defaultListBuilder(
     itemCount: blocks.length,
     padding: EdgeInsets.zero,
     itemBuilder: (final context, final index) =>
-        PortableTextConfig.shared.buildBlock(context, blocks[index]),
+        PortableTextConfig.of(context).buildBlock(context, blocks[index]),
   );
 }

packages/sanity/flutter_sanity_portable_text/pubspec.yaml

@@ -1,5 +1,4 @@
 name: flutter_sanity_portable_text
-resolution: workspace
 version: 1.8.1
 description: |
   Flutter renderer for the Sanity.io Portable Text format with support for