Firmware design-system handoff

Use this skill when a user has an existing firmware repo with a hand-rolled web UI (LittleFS / SPIFFS / similar partition, no build step on the device) and wants the design layer rebuilt as a real, token-driven design system that:

• ships as a drop-in package, not a refactor PR,
• backs up the existing data/ before touching anything,
• leaves behavior untouched (no JS rewrites beyond minimum hooks),
• comes with a visual regression page hosted on the device itself,
• is applyable on Windows with a double-click.

Inputs to gather (questions_v2 first)

Always ask these before writing anything:

1. Repo + screen list. Which firmware (path / repo URL), and which pages/screens does the UI have? Need: home, settings, log/monitor, one or more device-specific control panels, file explorer, etc.
2. Existing CSS state. Are there hand-rolled stylesheets per page, or one global CSS, and is there a dark-mode variant? Get the file names — they show up in the apply script's plan.
3. Theme toggle mechanism. body.dark class? [data-theme] attr? localStorage key? Match the existing convention so the toggle keeps working.
4. Chart library. ECharts? Chart.js? None? CDN or self-host? If CDN: which version is pinned. The skill emits a chart-theme helper that reads CSS vars regardless of library.
5. Fonts. Self-hosted WOFF2 (preferred for offline devices) or system stack? File-size budget if relevant — embedded partitions are usually 1-2 MB.
6. Variations to expose as Tweaks. Optional — usually no, this is production output, not a design exploration.
7. Scope. Just the design layer, or also a list of bugs/issues that need to be fixed alongside (e.g. "the X panel is broken").
8. Branding constraints. Which colors are "the brand"? What can be changed and what is sacred?
9. Iconography. Self-host PNG/SVG, icon font, or no icons?
10. Apply target. Windows / macOS / Linux for the script. Default to Windows since most firmware-flashing flows are Windows.

Package structure to emit

<project_name>_design_package/
├── README.md
├── handoff.md
├── SKILL.md            <-- this file, for next time
├── apply.ps1
├── apply.bat
├── data/               <-- mirrors firmware data/ structure
│   ├── ds-tokens.css
│   ├── components.css
│   ├── theme-toggle.js
│   ├── <chart>-theme.js
│   ├── design.html
│   └── fonts/
└── templates/
    ├── index.html.template
    └── <each-page>.html.template

Hard rules

• One tokens file. All color, type, space, radius, shadow, motion. Light + dark via the existing convention (body.dark AND [data-theme="dark"] for safety). No second source of truth.
• components.css uses var(--*) only. Zero raw hex outside ds-tokens.css. Search for #[0-9a-f] after writing — should match zero lines.
• Match existing class names. components.css must shadow the legacy CSS without markup changes for as many components as possible. The user removes the legacy CSS in the final patch, not the first.
• Sentinels in templates. Every patch block in templates/*.template is wrapped in <!-- DS:NAME:BEGIN --> … <!-- DS:NAME:END --> comments so the receiver knows exactly what to paste.
• Idempotent JS hooks. Use a data-bound="1" attribute on each bound element so re-running on a re-rendered DOM doesn't double-bind. Listen for a project-specific <page>:rendered event for re-binding.
• Chart theme reads tokens. Helper function calls getComputedStyle(document.body).getPropertyValue('--…') so theme swap is one chart.setOption(theme(), true) call.
• Backup before copy. apply.ps1 always writes a timestamped zip unless -NoBackup. Default mode is interactive (y/n/q per file); -Force skips prompts; -WhatIf is a dry run.
• No git, no build, no flash. Script copies files. handoff.md documents what's left for the receiver (Claude Code).
• /design.html lives on the device. Renders every component in light and dark side-by-side. Hosting it from the firmware means visual regression after each flash is one URL away.

Output discipline

• ds-tokens.css opens with a comment block explaining the dual-selector convention and the apply order.
• components.css has a numbered table-of-contents at the top.
• Each template's leading comment names the file it patches and the scope (full replace vs. block replace vs. append).
• handoff.md is structured as Patch A/B/C/D/E with explicit commit messages. The receiver should be able to land each as a separate PR if they want.
• README.md has Quick Start, Apply Order, Smoke Test, Rollback, "What the script does/doesn't do" — in that order.
• apply.ps1 uses [CmdletBinding(SupportsShouldProcess=$true)] so -WhatIf works without extra code.

Anti-patterns to avoid

• Self-hosting a chart library "for offline use" without checking with the user — it bloats the partition and is rarely worth it. Default to CDN, document the fallback.
• Hand-rolled icons. Use what's already in the firmware's assets/ or text labels. Don't invent SVG icons.
• Adding routes/handlers to the firmware's HTTP server. The design layer is files-only; routes are out of scope.
• Aggressive markup refactors. If a class name already exists, match it. The receiver should be able to land the package without rewriting partials.

When NOT to use this skill

• One-off styling tweaks (use a single PATCH.md instead).
• The user wants to explore design directions (use design_canvas).
• A from-scratch new firmware UI (build with the standard frontend design skill; this skill is for retrofitting an existing repo).