Theme Preview Sandbox
See a theme on the real components and real states before it becomes everyone's theme.
What it adds
A preview surface that renders the app's actual components, in every interaction state, under a theme that has not yet been saved.
What your agent is told to do
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What your agent is told to do
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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.
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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.
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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.
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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.
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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.
Edge cases it handles
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Edge cases it handles
7- 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.
Definition of done
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Definition of done
9- 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.
Related features
Z-Index Registry
Z-Index Registry
Keep every stacking value in one file so nobody has to guess a bigger number.
What it does
A single source of named z-index tokens, with reserved ranges per layer and a check that blocks raw values.
How it works
- 1 Find every z-index in the codebase, including inline styles and values buried in third-party overrides, and list them with the component that owns each. The count is usually higher than anyone expects and several will be arbitrary large numbers.
- 2 Reserve a range for each layer rather than assigning single values, so a new dropdown can slot between existing ones without renumbering everything above it. Leave real gaps between ranges.
- 3 Name each token after the layer it belongs to and derive the ordering from Elevation Layer Map. That feature decides what covers what; this one holds the numbers and stops them being invented locally.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/z-index-registry
Spacing Scale
Spacing Scale
Reduce the app's spacing to a small set of steps that repeat across every surface.
What it does
A named spacing scale covering gaps, padding, and section rhythm, with every existing value in the app mapped onto it.
How it works
- 1 Collect every spacing value currently in the codebase and count how often each appears. The result is usually a handful of genuine intentions surrounded by dozens of near-misses that were nudged once and never revisited.
- 2 Define a scale that is fine at the small end, where control padding lives, and coarse at the large end, where section separation lives. A linear scale gives too many indistinguishable large steps and too few small ones.
- 3 Separate the tokens used inside a component from the ones used between components and between page sections, and name them so the difference is obvious. The same number serving both purposes means a change to button padding also changes the gap between sections.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/spacing-scale
Shadow Scale
Shadow Scale
Replace ad hoc shadows with a small set tied to what each surface actually is.
What it does
A fixed ladder of shadow tokens, each named for an elevation role rather than a blur radius.
How it works
- 1 Collect every shadow currently declared anywhere in the app and group them by what they sit on — a resting card, a hovered card, a dropdown, a dialog, a toast. Most apps have a dozen near-identical values that collapse into four or five roles.
- 2 Name each step for its role, not its appearance. A token called menu tells the next person which surfaces may use it; a token called shadow-md-2 does not.
- 3 Use a single light source for the whole scale, so offset, blur, and spread grow together as elevation rises. Two shadows lit from different directions on the same screen read as a rendering bug.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/shadow-scale
How it works
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Copy the link
Grab the Markdown instruction URL for this feature.
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Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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It inspects, then implements
Your agent reads your existing app first, then adds the feature to fit it.
Works with your stack
These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.
Need it tighter than that? Customize the feature and tell it exactly what you're running.