Tailwind Theme Builder
Set up a fully themed Tailwind v4 + shadcn/ui project with dark mode. Produces configured CSS, theme provider, and working component library.
Architecture: The Four-Step Pattern
Tailwind v4 requires a specific architecture for CSS variable-based theming. This pattern is mandatory -- skipping or modifying steps breaks the theme.
How It Works
CSS Variable Definition --> @theme inline Mapping --> Tailwind Utility Class
--background --> --color-background --> bg-background
(with hsl() wrapper) (references variable) (generated class)
Dark mode switching:
ThemeProvider toggles .dark class on <html>
--> CSS variables update automatically (.dark overrides :root)
--> Tailwind utilities reference updated variables
--> UI updates without re-render
Best Practices
- Semantic names: Use
--primary not --blue-500
- Foreground pairing: Every background colour needs a foreground (
--primary + --primary-foreground)
- WCAG contrast: Normal text 4.5:1, large text 3:1, UI components 3:1
- Chart colours: Use separate variables with
@theme inline mapping, reference via var(--chart-1) in style props
Workflow
Step 1: Install Dependencies
pnpm add tailwindcss @tailwindcss/vite
pnpm add -D @types/node tw-animate-css
pnpm dlx shadcn@latest init
# Delete v3 config if it exists
rm -f tailwind.config.ts
Step 2: Configure Vite
Copy assets/vite.config.ts or add the Tailwind plugin:
import { defineConfig } from 'vite'
import react from '@vitejs/plugin-react'
import tailwindcss from '@tailwindcss/vite'
import path from 'path'
export default defineConfig({
plugins: [react(), tailwindcss()],
resolve: { alias: { '@': path.resolve(__dirname, './src') } }
})
Step 3: Four-Step CSS Architecture (Mandatory)
This exact order is required. Skipping steps breaks the theme.
src/index.css:
@import "tailwindcss";
@import "tw-animate-css";
/* 1. Define CSS variables at root (NOT inside @layer base) */
:root {
--background: hsl(0 0% 100%);
--foreground: hsl(222.2 84% 4.9%);
--primary: hsl(221.2 83.2% 53.3%);
--primary-foreground: hsl(210 40% 98%);
/* ... all semantic tokens */
}
.dark {
--background: hsl(222.2 84% 4.9%);
--foreground: hsl(210 40% 98%);
--primary: hsl(217.2 91.2% 59.8%);
--primary-foreground: hsl(222.2 47.4% 11.2%);
}
/* 2. Map variables to Tailwind utilities */
@theme inline {
--color-background: var(--background);
--color-foreground: var(--foreground);
--color-primary: var(--primary);
--color-primary-foreground: var(--primary-foreground);
}
/* 3. Apply base styles (NO hsl() wrapper here) */
@layer base {
body {
background-color: var(--background);
color: var(--foreground);
}
}
Result: bg-background, text-primary etc. work automatically. Dark mode switches via .dark class -- no dark: variants needed for semantic colours.
Step 4: Set Up Dark Mode
Copy assets/theme-provider.tsx to your components directory, then wrap your app:
import { ThemeProvider } from '@/components/theme-provider'
ReactDOM.createRoot(document.getElementById('root')!).render(
<ThemeProvider defaultTheme="dark" storageKey="vite-ui-theme">
<App />
</ThemeProvider>
)
Add a theme toggle -- install the dropdown menu then use the ModeToggle component below:
pnpm dlx shadcn@latest add dropdown-menu
// src/components/mode-toggle.tsx
import { Moon, Sun } from "lucide-react"
import { Button } from "@/components/ui/button"
import {
DropdownMenu,
DropdownMenuContent,
DropdownMenuItem,
DropdownMenuTrigger,
} from "@/components/ui/dropdown-menu"
import { useTheme } from "@/components/theme-provider"
export function ModeToggle() {
const { setTheme } = useTheme()
return (
<DropdownMenu>
<DropdownMenuTrigger asChild>
<Button variant="outline" size="icon">
<Sun className="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
<Moon className="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
<span className="sr-only">Toggle theme</span>
</Button>
</DropdownMenuTrigger>
<DropdownMenuContent align="end">
<DropdownMenuItem onClick={() => setTheme("light")}>Light</DropdownMenuItem>
<DropdownMenuItem onClick={() => setTheme("dark")}>Dark</DropdownMenuItem>
<DropdownMenuItem onClick={() => setTheme("system")}>System</DropdownMenuItem>
</DropdownMenuContent>
</DropdownMenu>
)
}
Step 5: Configure components.json
{
"tailwind": {
"config": "",
"css": "src/index.css",
"baseColor": "slate",
"cssVariables": true
}
}
"config": "" is critical -- v4 doesn't use tailwind.config.ts.
Critical Rules
Always:
- Wrap colours with
hsl() in :root/.dark
- Use
@theme inline to map all CSS variables
- Use
@tailwindcss/vite plugin (NOT PostCSS)
- Delete
tailwind.config.ts if it exists
Never:
- Put
:root/.dark inside @layer base
- Use
.dark { @theme { } } (v4 doesn't support nested @theme)
- Double-wrap:
hsl(var(--background))
- Use
@apply with @layer base classes (use @utility instead)
All 18 Gotchas
Quick Diagnosis
| # | Symptom | Cause | Fix |
|---|
| 1 | Variables ignored / theme broken | :root inside @layer base | Move :root and .dark to root level |
| 2 | Dark mode colours not switching | .dark { @theme { } } | Use CSS variables + single @theme inline |
| 3 | Colours all black/white | Double hsl() wrapping | Use var(--background) not hsl(var(...)) |
| 4 | bg-primary not generated | Colours in tailwind.config.ts | Delete config, use @theme inline |
| 5 | bg-background class missing | No @theme inline block | Add @theme inline mapping variables |
| 6 | shadcn components break | components.json has config path | Set "config": "" (empty string) |
| 7 | Tailwind not processing | Using PostCSS plugin | Switch to @tailwindcss/vite plugin |
| 8 | @/ imports fail | Missing path aliases | Add paths to tsconfig.app.json |
| 9 | Redundant dark: variants | Using dark:bg-primary-dark | Just use bg-primary -- variables handle it |
| 10 | Hardcoded colours everywhere | Using bg-blue-600 dark:bg-blue-400 | Use semantic tokens: bg-primary |
| 11 | Class merging bugs | String concatenation for classes | Use cn() from @/lib/utils |
| 12 | Radix Select crashes | Empty string value value="" | Use value="placeholder" |
| 13 | Wrong Tailwind version | Installed tailwindcss@^3 | Install tailwindcss@^4.1.0 + @tailwindcss/vite |
| 14 | Missing peer deps | Only installed tailwindcss | Also install clsx, tailwind-merge, @types/node |
| 15 | Broken in dark mode | Only tested light mode | Test light, dark, system, and toggle transitions |
| 16 | Fails WCAG contrast | Looks fine visually | Check ratios: 4.5:1 normal text, 3:1 large/UI |
| 17 | Build fails on animation import | Using tailwindcss-animate (deprecated) | Use tw-animate-css or native CSS animations |
| 18 | CSS priority issues | Duplicate @layer base after shadcn init | Merge into single @layer base block |
Gotcha Details with Code Examples
#1 -- :root inside @layer base
Tailwind v4 strips CSS outside @theme/@layer, but :root must be at root level to persist. This is the most common setup failure.
WRONG:
@layer base {
:root { --background: hsl(0 0% 100%); }
}
CORRECT:
:root { --background: hsl(0 0% 100%); }
@layer base {
body { background-color: var(--background); }
}
#2 -- Nested @theme
Tailwind v4 does not support @theme inside selectors. Use CSS variables in :root/.dark with a single @theme inline block.
WRONG:
@theme { --color-primary: hsl(0 0% 0%); }
.dark { @theme { --color-primary: hsl(0 0% 100%); } }
CORRECT:
:root { --primary: hsl(0 0% 0%); }
.dark { --primary: hsl(0 0% 100%); }
@theme inline { --color-primary: var(--primary); }
#3 -- Double hsl() wrapping
Variables already contain hsl(). Double-wrapping creates hsl(hsl(...)).
WRONG: background-color: hsl(var(--background));
CORRECT: background-color: var(--background);
#4 -- Colours in tailwind.config.ts
Tailwind v4 completely ignores theme.extend.colors in config files. Delete the file or leave it empty. Set "config": "" in components.json.
#5 -- Missing @theme inline
Without @theme inline, Tailwind has no knowledge of your CSS variables. Utility classes like bg-background simply won't be generated.
WRONG:
:root { --background: hsl(0 0% 100%); }
/* No @theme inline block -- bg-background won't exist */
CORRECT:
:root { --background: hsl(0 0% 100%); }
@theme inline { --color-background: var(--background); }
#7 -- PostCSS vs Vite plugin
WRONG:
export default defineConfig({
css: { postcss: './postcss.config.js' } // Old v3 way
})
CORRECT:
import tailwindcss from '@tailwindcss/vite'
export default defineConfig({
plugins: [react(), tailwindcss()] // v4 way
})
#8 -- Path aliases
Add to tsconfig.app.json:
{
"compilerOptions": {
"baseUrl": ".",
"paths": { "@/*": ["./src/*"] }
}
}
#11 -- cn() utility for class merging
WRONG: className={`base ${isActive && 'active'}`}
CORRECT: className={cn("base", isActive && "active")}
cn() from @/lib/utils properly merges and deduplicates Tailwind classes.
#12 -- Radix Select empty value
Radix UI Select does not allow empty string values. Use value="placeholder" instead of value="".
#14 -- Required dependencies
{
"dependencies": {
"tailwindcss": "^4.1.0",
"@tailwindcss/vite": "^4.1.0",
"clsx": "^2.1.1",
"tailwind-merge": "^3.3.1"
},
"devDependencies": {
"@types/node": "^24.0.0"
}
}
#17 -- tw-animate-css
tailwindcss-animate is deprecated in Tailwind v4. shadcn/ui docs may still reference it. Causes build failures and import errors. Use tw-animate-css or @tailwindcss/motion instead.
#18 -- Duplicate @layer base after shadcn init
shadcn init adds its own @layer base block. Check src/index.css immediately after running init and merge any duplicate blocks into one.
WRONG:
@layer base { body { background-color: var(--background); } }
@layer base { * { border-color: hsl(var(--border)); } } /* duplicate from shadcn */
CORRECT:
@layer base {
* { border-color: var(--border); }
body { background-color: var(--background); color: var(--foreground); }
}
Prevention Checklist
Dark Mode Testing Checklist
Asset Files
Copy from assets/ directory:
index.css -- Complete CSS with all colour variablescomponents.json -- shadcn/ui v4 configvite.config.ts -- Vite + Tailwind plugintheme-provider.tsx -- Dark mode providerutils.ts -- cn() utility
Reference Files
references/migration-guide.md -- v3 to v4 migration
Official Documentation