Theming: Storybook Appearance Integration

Summary

Extend StorybookTheme.tsx to support 'dynamic' system appearance and a
Flex-authored theme, replace hard-coded hex values in the toolbar chrome with
token-derived values, and add deterministic tests that exercise theme selection
and validate toolbar behavior on both macOS and Windows.

The Storybook toolbar is the only runtime surface that exercises the full
theme-switching flow. Its current four choices all use createDefaultTheme, it
cannot exercise 'dynamic' appearance or the Flex-value context, and the
toolbar styles are literal hex values that are inconsistent with the theming
patterns the workstream is establishing.

Goal

After this task, the Storybook toolbar adds a 'dynamic' system-appearance
option, a Flex-authored-theme option, and token-sourced toolbar chrome. Tests
confirm that each toolbar choice produces the expected context value, that the
toolbar is functional on macOS and Windows, and that CI does not depend on the
host machine's system appearance.

Stage

Stage 2 - Beta delivery.

Why it matters

• Observed. The toolbar hard-codes four createDefaultTheme calls and has
  no 'dynamic' option, so a developer running the Storybook cannot trigger the
  system-appearance code path without modifying source
  (apps/storybook/src/StorybookTheme.tsx).
• Observed. No toolbar option exercises the Flex-value context. Every choice
  is a ThemeReference built from a Fluent Theme, so a Flex-authored theme
  has no integration exercise in the Storybook today.
• Observed. The toolbar StyleSheet contains eleven literal hex values
  (#f5f5f5, #d1d1d1, #242424, #ffffff, #b3b3b3, #0f6cbd) that
  duplicate design token values rather than sourcing them from the token system
  (StorybookTheme.tsx styles block).
• Observed. The Windows smoke test
  (apps/storybook/windows-tests/storybook-smoke.test.cjs) does not exercise
  any theme button, so the toolbar is not covered by automated validation on
  Windows.
• Inferred. Without a 'dynamic' integration test, a regression in
  appearance-change notification or ThemeState invalidation on macOS or
  Windows could ship silently.
• Inferred. Toolbar colors derived from literals are inconsistent with
  themedStyleSheetFactory usage across agentic components and create a
  maintenance burden when brand or neutral token values change.

Observed current state

• Observed. themeChoiceNames is ['none', 'light', 'dark', 'highContrast'].
  themeChoices builds each entry with createDefaultTheme from
  @fluentui-react-native/default-theme; 'none' omits the theme entirely
  (StorybookTheme.tsx lines 12-20).
• Observed. StorybookThemeProvider (used as a story decorator in
  preview.tsx)
  reads the ThemeReference from StorybookThemeContext and conditionally
  renders <ThemeProvider theme={theme}>. No Flex-value context provider is
  present.
• Observed. StorybookThemeHost in
  StorybookApp.tsx
  wraps the entire Storybook UI and is the root that renders the toolbar.
  The toolbar sits outside the story-level ThemeProvider, so it currently has
  no access to the Flex tokens of the selected theme.
• Observed. Toolbar testID values are already stable:
  agentic-storybook-theme-toolbar on the containing View and
  agentic-storybook-theme-${name} on each button, providing reliable
  automation targets for both macOS accessibility IDs and Windows UIA selectors.
• Observed. The Windows smoke test locates story elements by testID using
  @react-native-windows/automation but makes no reference to any theme button
  (storybook-smoke.test.cjs).
• Observed. useFlexTokens currently ignores context and always returns
  defaultFlexTokens
  (packages/agentic/design/src/tokens/useFlexTokens.ts);
  this is noted as temporary in
  PR #4156
  and is the subject of Dynamic Theme Building.
• Observed. System appearance handling is split across four platformUtils
  files and ThemeState does not expose the resolved scheme; this is the
  subject of System Appearance Handling.

Scope

• Add a 'dynamic' toolbar option that constructs a theme using the
  'dynamic' appearance value and relies on the platform subscription defined
  in System Appearance Handling to re-render when the
  system appearance changes.
• Add a Flex theme toolbar option that wraps stories with the Flex-value context
  provider defined in Dynamic Theme Building,
  supplying a minimal Flex-authored token set so the full context path is
  exercised without a Fluent Theme present.
• Replace the literal hex values in the toolbar StyleSheet with values sourced
  from the Flex token set. The toolbar chrome must remain functional when no
  theme is selected (the 'none' case), so it requires a stable base token set
  that does not depend on a story-level ThemeProvider.
• Add unit tests (Jest, host environment) that render StorybookThemeHost and
  assert: each button sets the expected context value, StorybookThemeProvider
  wraps with ThemeProvider only when a theme is selected, and selecting the
  Flex option activates the Flex-value context instead of ThemeProvider.
• Extend the Windows smoke test to locate and activate at least the light and
  dark toolbar buttons by their stable testID values and confirm the toolbar
  is present and interactive after each selection.
• Document the macOS validation steps: verifying that selecting 'dynamic'
  with macOS system appearance toggled between light and dark re-renders the
  story with updated tokens.
• Provide a deterministic CI strategy so 'dynamic' appearance selections do
  not make snapshot or token-equality tests dependent on the host machine's
  system appearance setting.

Out of scope

• Implementing 'dynamic' appearance resolution and the platform subscription
  mechanism; see System Appearance Handling.
• Defining the Flex-value context type and making useFlexTokens context-aware;
  see Dynamic Theme Building.
• Generating default Flex token values or adding appearance-specific token sets;
  see Default Values Codegen.
• Building a macOS or Windows platform theme against the Flex contract; see
  Apple Theme.

Deliverables

1. An extended themeChoiceNames tuple that includes 'dynamic' and a Flex
   theme option, with corresponding themeChoices entries.
2. A token-sourced toolbar StyleSheet that reads chrome colors from the Flex
   token set, with a documented fallback for the 'none' case.
3. A Flex-value context provider path in StorybookThemeProvider that activates
   when the Flex theme option is selected.
4. Unit tests covering all toolbar choices, context wrapping, and the Flex
   context path.
5. An extended Windows smoke test that exercises the theme toolbar buttons.
6. A macOS validation note documenting the manual appearance-toggle verification
   steps and the expected re-render behavior.
7. A CI determinism note or implementation specifying how 'dynamic' is handled
   in tests (for example, forcing a fixed system appearance in Jest or skipping
   appearance-dependent assertions).
8. Changesets.

Acceptance criteria

• The toolbar exposes a 'dynamic' option that activates a theme constructed
  with appearance: 'dynamic', and a system appearance change on macOS
  re-renders the story without a crash.
• The toolbar exposes a Flex theme option that supplies tokens through the
  Flex-value context defined in Dynamic Theme Building
  without constructing a Fluent Theme.
• The toolbar StyleSheet contains no hex literals; all chrome colors are
  derived from the Flex token set, and the 'none' case renders the toolbar
  with a stable default without accessing an undefined token.
• Existing testID values (agentic-storybook-theme-toolbar,
  agentic-storybook-theme-none, agentic-storybook-theme-light,
  agentic-storybook-theme-dark, agentic-storybook-theme-highContrast,
  plus any new entries) remain stable and are findable by Windows UIA and
  macOS accessibility.
• Unit tests pass: each toolbar choice produces the correct context value,
  StorybookThemeProvider wraps with ThemeProvider for Fluent choices and
  with the Flex context provider for the Flex choice, and 'none' renders
  children without either.
• The Windows smoke test locates the theme toolbar, activates the light and
  dark buttons, and confirms the toolbar element remains displayed after each
  selection.
• CI test runs do not fail or produce different results depending on the host
  machine's system appearance setting.
• yarn build, yarn lage test, and yarn lage lint pass at the
  repository root, and changesets are present.

Dependencies and ordering

Depends on Dynamic Theme Building (Stage 1) for
the Flex-value context type and the context-aware useFlexTokens. The Flex
theme option cannot be wired until that context exists and a minimal Flex token
set can be supplied.

Depends on System Appearance Handling (Stage 2) for
the 'dynamic' appearance value to be reliably resolved into a concrete scheme.
The dynamic toolbar option can be scaffolded before that task lands, but its
subscription behavior and the macOS and Windows validation steps cannot be
completed until the structured appearance model is in place.

Should follow Package Consolidation so the Flex
context type and the toolbar's token imports resolve from their final submodule
paths in @fluentui-react-native/design.

Risks and open decisions

• Open decision. Which Flex token set the toolbar chrome uses when 'none'
  is selected. Observed: defaultFlexTokens is always available as the
  module-level default in defaultTokens.ts, so it is the natural fallback.
  The owner should confirm whether the chrome should always render with the
  default Flex tokens or should follow whatever the selected theme supplies.
• Open decision. Whether the Flex toolbar option uses the generated default
  Flex tokens or a small authored override set. Inferred: using the generated
  defaults keeps the option stateless but exercises less of the override path;
  a small authored set with a visible brand color change would make the Flex
  context visually distinguishable from the existing 'light' choice. This
  interacts with Default Values Codegen.
• Open decision. Whether 'dynamic' should be disabled or hidden on
  platforms where the system appearance subscription is not available.
  Observed: getCurrentAppearance on the default platform falls back to
  the defaultAppearance value, so a 'dynamic' theme is safe to construct on
  all platforms but may not respond to system changes everywhere.
• Open decision. How to make CI deterministic for the 'dynamic' option.
  Options include: (a) mock Appearance.getColorScheme to return 'light' in
  Jest, (b) exclude 'dynamic' from snapshot tests and cover it only with
  interaction tests that do not assert token values, or (c) use a controlled
  ThemeReference that forces a fixed appearance in test environments. The
  chosen approach must be documented in the test file.
• Risk. Token-sourced chrome requires useFlexTokens (or useThemeState)
  to be callable outside the story-level ThemeProvider. If the chrome hook is
  called from StorybookThemeHost before any ThemeProvider is mounted, it
  must safely return the default token set; the current context-free
  implementation in useFlexTokens.ts already does this, but the updated
  context-aware version must preserve that fallback.
• Risk. The toolbar sits outside StorybookThemeProvider, which is the
  story-level decorator. Adding a Flex context provider there does not affect the
  toolbar, so the two providers are structurally separate. A design where the
  chrome uses story-level tokens (the Flex choice) would require re-ordering the
  provider hierarchy, which may affect story rendering.

Evidence and references

• apps/storybook/src/StorybookTheme.tsx: current toolbar implementation with four fixed choices and literal hex styles.
• apps/storybook/src/preview.tsx: StorybookThemeProvider used as a story-level decorator.
• apps/storybook/src/StorybookApp.tsx: StorybookThemeHost as the root wrapper.
• apps/storybook/windows-tests/storybook-smoke.test.cjs: Windows automation test that exercises story testID values but not the theme toolbar.
• apps/storybook/scripts/smoke-stories.json: smoke-test story index.
• packages/agentic/design/src/tokens/useFlexTokens.ts: temporary context-free implementation; context-awareness is the subject of Dynamic Theme Building.
• packages/agentic/design/src/useThemeState.ts: ThemeState, caching, and themedStyleSheetFactory.
• packages/agentic/design/src/tokens/defaultTokens.ts: defaultFlexTokens as the fallback token set.
• packages/agentic/design/src/theming/types/Theme.types.ts: AppearanceOptions including 'dynamic'.
• System Appearance Handling: owns 'dynamic' resolution and the platform subscription model that this task's dynamic toolbar option depends on.
• Dynamic Theme Building: owns the Flex-value context type and context-aware useFlexTokens that this task's Flex theme option depends on.
• Package Consolidation: defines the final submodule paths that this task imports from.
• Default Values Codegen: determines what the default Flex token set contains for the 'none' chrome fallback and potentially the Flex theme option.

Start with apps/storybook/src/StorybookTheme.tsx, then trace StorybookThemeProvider in preview.tsx and StorybookThemeHost in StorybookApp.tsx. Read useFlexTokens.ts and the linked dependency issues before proceeding, and run apps/storybook/windows-tests/storybook-smoke.test.cjs. Done means all toolbar choices and context paths are tested, Windows light/dark interaction works, dynamic behavior is deterministic, macOS validation is documented, root checks pass, and changesets are present.