# Theme Preview Sandbox

## Objective

See a theme on the real components and real states before it becomes everyone's theme.

A preview surface that renders the app's actual components, in every interaction state, under a theme that has not yet been saved.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Render the app's real components in the preview, imported from the same place the product screens import them. Swatch grids prove nothing — a palette only fails once it is a disabled button on a card inside a dialog.
2. Show every interaction state at once: default, hover, active, focus, selected, disabled, loading, and error. The state colours themselves are owned by State Color Matrix; the sandbox is where you check that matrix against a candidate theme.
3. Include the surfaces where themes usually break — charts, dense tables, form fields with validation, overlays and their backdrops, and anything with a shadow or a translucent panel.
4. Scope the candidate theme to the preview only. It must never be written to the saved settings, must not leak into other open tabs, and must be discarded on navigating away unless it is explicitly applied.
5. Offer light and dark side by side rather than behind a toggle, so the difference between the two is visible in one glance instead of held in memory. Do not build a separate stripped-down copy of each component for the sandbox; a preview of components that are not the real ones is worse than no preview.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- The candidate theme must be isolated from saved settings. A preview that writes through means someone experimenting with a colour has already changed what every user sees.
- Hover, focus, disabled, and error states must all be rendered simultaneously and without requiring interaction, because the states nobody can be bothered to reproduce by hand are exactly the ones that fail.
- Charts, tables, forms, and overlays have to appear in the preview. A theme that looks correct on buttons routinely falls apart on a stacked bar chart or a striped table.
- Light and dark must be viewable side by side, since a theme is usually broken in only one of the two and toggling hides the comparison.
- Applying a preview must be a distinct, deliberate action with a clear result, and abandoning the preview must leave no residue in local storage or the URL.
- Contrast verification belongs to the In-App Contrast Checker — embed that checker in the sandbox rather than writing a second measurement implementation that will disagree with the first.
- A theme that fails to parse or is missing tokens must render as a clear error in the sandbox, not fall back silently to the current theme and appear to pass.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] The preview renders the same component implementations the product uses.
- [ ] Default, hover, active, focus, selected, disabled, loading, and error states are all visible without interaction.
- [ ] Charts, tables, form validation, and overlays are represented.
- [ ] Light and dark render side by side.
- [ ] A previewed theme is never persisted until it is explicitly applied, and leaves no residue when abandoned.
- [ ] Contrast results in the sandbox come from the existing checker, not a second implementation.
- [ ] An invalid or incomplete theme surfaces as an error rather than falling back silently.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.