Tailwind CSS v4 + shadcn/ui integration

Technical reference for how PolicyEngine's CSS-first design token architecture works. This skill explains the mechanism — how Tailwind v4 and shadcn/ui consume CSS variables. For the actual token values (colors, fonts, spacing), see policyengine-design-skill.

Architecture overview

PolicyEngine uses a single CSS file (@policyengine/ui-kit/theme.css) as the source of truth for all design tokens. This file has three layers:

Layer 1: :root { --primary: #2C7A7B; }          ← Raw values (shadcn/ui convention)
Layer 2: @theme inline { --color-primary: var(--primary); }  ← Bridge to Tailwind
Layer 3: @theme { --color-teal-500: #319795; }   ← Brand palette + fonts + sizes

Consumers import it in their globals.css:

@import "tailwindcss";
@import "@policyengine/ui-kit/theme.css";

Tailwind v4 @theme namespaces

Tailwind v4 uses CSS custom properties in @theme blocks to generate utility classes. The namespace prefix determines which utilities are created:

Source: Tailwind v4 Theme docs

@theme vs @theme inline

/* @theme — bakes values directly into generated CSS */
@theme {
  --color-teal-500: #319795;  /* Resolved at build time */
}

/* @theme inline — preserves var() for runtime resolution */
@theme inline {
  --color-primary: var(--primary);  /* Resolved at runtime via :root */
}

When to use @theme inline:

• When the value references a :root CSS variable (var(--something))
• Required for dark mode (:root values change, Tailwind must re-resolve)
• Used for all shadcn/ui semantic tokens (primary, background, foreground, etc.)

When to use @theme:

• When the value is a static literal (#319795, 14px, Inter)
• Used for brand palette colors, font sizes, spacing, breakpoints

Source: Tailwind v4 Functions and Directives

shadcn/ui CSS variable convention

shadcn/ui defines semantic tokens as unprefixed CSS variables in :root:

:root {
  --primary: #2C7A7B;
  --background: #FFFFFF;
  --foreground: #000000;
  --muted: #F2F4F7;
  --muted-foreground: #6B7280;
  --border: #E2E8F0;
  --chart-1: #319795;
  --chart-2: #0EA5E9;
  /* ... */
}

These are then bridged to Tailwind via @theme inline:

@theme inline {
  --color-primary: var(--primary);       /* → bg-primary, text-primary */
  --color-background: var(--background); /* → bg-background */
  --color-foreground: var(--foreground); /* → text-foreground */
  --color-chart-1: var(--chart-1);       /* → fill-chart-1 */
}

Source: shadcn/ui Theming, shadcn/ui Tailwind v4 guide

SVG var() in Recharts

Modern browsers resolve CSS custom properties in SVG presentation attributes. Recharts fill and stroke props accept var() directly:

<Bar fill="var(--chart-1)" />
<Line stroke="var(--chart-2)" />
<CartesianGrid stroke="var(--border)" />

No helper function needed. The old getCssVar() / getComputedStyle() pattern is unnecessary.

Source: shadcn/ui Charts

Complete namespace reference

Layer 1: :root (shadcn/ui semantic)

Layer 2: @theme inline (bridges)

Maps each :root var to Tailwind's --color-* namespace. Example:

• --color-primary: var(--primary) → enables bg-primary, text-primary
• --color-chart-1: var(--chart-1) → enables fill-chart-1

Layer 3: @theme (brand palette)

Common mistakes

1. Using @theme instead of @theme inline for var() references

/* WRONG — var() won't resolve, Tailwind bakes the literal string */
@theme {
  --color-primary: var(--primary);
}

/* CORRECT — var() resolves at runtime */
@theme inline {
  --color-primary: var(--primary);
}

2. Wrong namespace prefix

/* WRONG — creates a utility called "primary", not a color */
@theme {
  --primary: #2C7A7B;
}

/* CORRECT — --color-* prefix creates bg-primary, text-primary */
@theme inline {
  --color-primary: var(--primary);
}

3. Using getCssVar() for Recharts

// WRONG — unnecessary helper
const color = getCssVar('--chart-1');
<Bar fill={color} />

// CORRECT — SVG accepts var() directly
<Bar fill="var(--chart-1)" />

4. Hardcoding hex in components

// WRONG
<div style={{ color: '#319795' }}>

// CORRECT — use Tailwind class
<div className="text-teal-500">

// CORRECT — use CSS var for inline styles
<div style={{ color: 'var(--primary)' }}>

5. Creating tailwind.config.ts

Tailwind v4 does NOT use tailwind.config.ts. All configuration is in CSS via @theme blocks. The ui-kit theme CSS file handles this.

6. Using old pe-* prefixed classes

// WRONG — old convention
<div className="bg-pe-primary-500 text-pe-text-secondary p-pe-lg">

// CORRECT — standard Tailwind/shadcn classes
<div className="bg-teal-500 text-muted-foreground p-4">

Related skills

• policyengine-design-skill — Token values, color tables, chart branding
• policyengine-frontend-builder-spec-skill — Mandatory technology requirements
• policyengine-recharts-skill — Chart-specific patterns
• policyengine-interactive-tools-skill — Standalone tool scaffolding