@layer base {
  :root {
    --background: 0 0% 100%;
    --foreground: 222.2 84% 4.9%;
    --primary: 222.2 47.4% 11.2%;
    --primary-foreground: 210 40% 98%;
    --secondary: 210 40% 96.1%;
    --muted: 210 40% 96.1%;
    --accent: 210 40% 96.1%;
    --danger: 0 84.2% 60.2%;
    --border: 214.3 31.8% 91.4%;
    --radius: 0.5rem;
  }

  .dark {
    --background: 222.2 84% 4.9%;
    --foreground: 210 40% 98%;
    /* ... dark mode values */
  }
}

Adding Custom Colors

// tailwind.config.ts
export default {
  theme: {
    extend: {
      colors: {
        brand: {
          primary: "#0070F3",
          secondary: "#7928CA",
        },
      },
    },
  },
};

Adding Animations

theme: {
  extend: {
    keyframes: {
      shimmer: {
        "0%": { backgroundPosition: "-200% 0" },
        "100%": { backgroundPosition: "200% 0" },
      },
    },
    animation: {
      shimmer: "shimmer 2s linear infinite",
    },
  },
}

Custom Utilities

/* globals.css */
@layer utilities {
  .text-balance {
    text-wrap: balance;
  }
  .scrollbar-hidden {
    -ms-overflow-style: none;
    scrollbar-width: none;
  }
}

Plugins

plugins: [
  require("@tailwindcss/typography"),  // prose classes
  require("@tailwindcss/forms"),        // form resets
]

Size Utility Convention

Use size-* instead of h-* w-* for equal dimensions:

// ✅ Good
<div className="size-4">Icon</div>
<div className="size-8">Avatar</div>

// ❌ Avoid
<div className="h-4 w-4">Icon</div>

Dark Mode

Tailwind v4 uses a CSS custom variant instead of config-based darkMode:

/* globals.css */
@custom-variant dark (&:is(.dark *));

This is configured in apps/web/src/app/globals.css, which also defines dark CSS variables.

Theme switching uses next-themes with attribute="class" to toggle .dark on <html>:

import { useTheme } from "next-themes";
const { resolvedTheme, setTheme } = useTheme();

Status: Dark mode CSS infrastructure is ready via web-local theme tokens.

Debugging

# Check resolved config
npx tailwindcss config

# Watch for generated classes
TAILWIND_MODE=watch npx tailwindcss -i ./src/styles/globals.css -o ./dist/output.css --watch

Common issues:

Classes not applying → Check content paths
Dark mode not working → Verify darkMode: ["class"] and ThemeProvider
Classes purged → Add to safelist or use complete class names

Best Practices

Mobile-first - Default styles for mobile, add breakpoints up
CSS Variables - Use for theming (enables dark mode)
Content paths - Include all component paths
Semantic names - Use brand, accent not blue-500
Size utility - Use size-* for equal dimensions

References

Tailwind CSS: https://tailwindcss.com
Design tokens: See design-language-system skill

tailwind-config

Tailwind CSS Configuration Skill

Configuration Structure

CSS Variables