21st-design-sync

21st Design Sync — publish your project's design as a theme

Take the design a project already ships (its light + dark CSS variables) and publish it to the public 21st.dev theme library so anyone can preview, bookmark, and apply it. Under the hood this is one CLI call: 21st publish-theme <file.css> --name "…".

Pre-flight (always)

1. Auth needs a real API key. publish-theme is a management endpoint: it accepts a 21st_sk_… key only, not a 21st login session token. Get one at https://21st.dev/mcp (or https://21st.dev/settings/api-keys) and pass it via --api-key 21st_sk_… or the TWENTYFIRST_TOKEN / API_KEY_21ST env var. If the user has no key, point them there — don't try to mint one.
2. The CLI is the unified @21st-dev/cli (bin 21st). Use npx @21st-dev/cli if it isn't installed.
3. Publishing is public and outward-facing. A published theme is immediately is_public in the community library (there is no unlisted/private option for themes). Confirm with the user before publishing.

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Step 1 — Find the project's design tokens

Locate the file that defines the shadcn/Tailwind theme variables. Check, in order:

• app/globals.css, src/app/globals.css
• src/index.css, styles/globals.css, app/styles/globals.css

You're looking for a :root { … } block of CSS custom properties (--background, --foreground, --card, --primary, --secondary, --muted, --accent, --destructive, --border, --input, --ring, the --chart-* / --sidebar-* tokens, --radius) and a matching .dark { … } block. Values may be hsl(...), oklch(...), hex, or raw channels — keep them exactly as the project wrote them.

Tailwind v4 projects usually keep the same :root / .dark blocks plus an @theme inline mapping; you only need the :root and .dark token values, not the @theme mapping.

Step 2 — Assemble a valid theme CSS file

The publish endpoint parses --name: value; pairs out of a :root { … } block and a .dark { … } block, and requires both to be non-empty. So the file you publish must contain both.

• Copy the project's :root and .dark blocks into a standalone file (e.g. project-theme.css). Nothing else is required.
• If the project has only a light :root and no .dark (or an empty one), generate a dark variant before publishing — don't ship a theme with an empty dark mode. Reuse the add-dark-mode / oklch-skill approach: invert lightness while preserving hue/chroma so the dark set stays on-brand. Keep the same token names.
• Keep the token names shadcn-standard so the theme previews correctly on the card and applies cleanly for others.

Minimal shape:

:root {
  --background: 0 0% 100%;
  --foreground: 240 10% 4%;
  --primary: 240 6% 10%;
  /* …the rest of the project's light tokens… */
  --radius: 0.5rem;
}
.dark {
  --background: 240 10% 4%;
  --foreground: 0 0% 98%;
  --primary: 0 0% 98%;
  /* …the rest of the project's dark tokens… */
}

Step 3 — Name & tag it

• Name (required, ≤ 50 chars): infer from the project — the product/brand name from package.json, the repo, or the site title. Ask the user if it's ambiguous.
• Tags (optional): a few descriptors that match how people browse themes, e.g. dark, minimal, neutral, vibrant, saas. Pass comma-separated.

Step 4 — Publish

21st publish-theme ./project-theme.css \
  --name "Acme" \
  --tags dark,minimal \
  --api-key 21st_sk_…            # or set TWENTYFIRST_TOKEN

The command prints the live theme URL (https://21st.dev/community/themes/<slug>). Share it with the user.

Updating vs re-publishing

• Each publish-theme creates a NEW theme (there's no upsert-by-slug like components have). Running it twice = two themes in the library.
• To change name/tags/visibility on an existing theme, edit it in place: 21st edit <theme-id> --type theme [--name "…"] [--tags a,b].
• To change the colors, publish a fresh file and remove the old one: 21st delete <theme-id> --type theme --yes (soft-unpublish, reversible).

When NOT to use this

• Syncing a component library to a Claude Design project → that's the built-in /design-sync skill, a different destination.
• Publishing a component (not a color theme) → use 21st-registry.