# Prompt to Add Dark Mode with Tailwind CSS v4

> AI agent prompt to add system-aware dark mode to a Next.js app using Tailwind CSS v4 CSS variables and next-themes, with no flash on load.

**Type:** Prompt  
**Tools:** Cursor, Claude Code, Codex, Windsurf  
**Stack:** Next.js, TypeScript, Tailwind  
**Difficulty:** easy  
**Updated:** 2026-06-08

---

Give this prompt to your agent to add dark mode using Tailwind v4's CSS-variable-based
theming and `next-themes` — preventing the flash of unstyled content that plagues naive
`localStorage` implementations.

## Main Prompt

```txt title="Main Prompt"
You are working in a Next.js App Router project with TypeScript and Tailwind CSS v4.

Task: add dark mode with system preference detection and a manual toggle.

Requirements:
- Install `next-themes`.
- In `src/app/layout.tsx`, wrap the app in `<ThemeProvider attribute="class" defaultTheme="system" enableSystem>`.
  The `ThemeProvider` is a Client Component — create a wrapper `src/components/Providers.tsx` if
  the root layout must remain a Server Component.
- In `src/styles/global.css`, define CSS custom properties for both themes using the Tailwind v4
  `@theme` directive:
    - Light: `--color-bg: oklch(100% 0 0)`, `--color-text: oklch(9% 0 0)`.
    - Dark (inside `[class="dark"]` selector): override those variables.
  Apply them to the body as `background: var(--color-bg)` and `color: var(--color-text)`.
- Create `src/components/ThemeToggle.tsx` (Client Component) using `useTheme` from `next-themes`.
  It should cycle through light → dark → system on click and show a sun/moon/monitor icon.
  Use Tailwind's built-in `dark:` variant for icon visibility — do NOT use conditional classNames
  based on the `theme` string for icon color.
- Do not use `tailwind.config.ts` `darkMode: 'class'` — that is Tailwind v3 API. In v4, the
  `dark:` variant is class-based by default.
- Do NOT install lucide-react or any icon library — use SVG inline.

Stop and list all planned file changes before writing any code.
```

## Implementation Notes

- In Tailwind v4, dark mode via `dark:` prefix works with the `.dark` class on `html` automatically
  — no config entry is needed.
- `next-themes` reads the saved theme from `localStorage` and sets the class before first paint,
  which prevents the FOUC.
- The `ThemeProvider` must be client-side, but the root `layout.tsx` can stay server-side by
  extracting the provider to a `Providers.tsx` wrapper component marked `'use client'`.

## Expected File Changes

```txt
src/app/layout.tsx                     (edited — add Providers wrapper)
src/components/Providers.tsx           (new — Client Component with ThemeProvider)
src/components/ThemeToggle.tsx         (new — Client Component)
src/styles/global.css                  (edited — CSS custom properties for light/dark)
package.json                           (edited — add next-themes)
```

## Acceptance Criteria

- On first load with system set to dark, the page renders dark without a flash.
- Toggling ThemeToggle cycles through light, dark, and system modes; the preference persists on reload.
- All existing `dark:` Tailwind utilities work without any config changes.
- `bun run build` exits 0.

## Test Commands

```bash
bun add next-themes
bun run typecheck
bun run build
bun run dev
# open DevTools, set system dark mode, reload — confirm no white flash
# click ThemeToggle three times and confirm cycle: light → dark → system
```

## Common AI Mistakes

- Adding `darkMode: 'class'` to a `tailwind.config.ts` that doesn't exist in a v4 project.
- Wrapping the root Server Component `layout.tsx` with `'use client'` instead of extracting a Providers component.
- Using `theme === 'dark'` conditional rendering instead of Tailwind's `dark:` variant, causing hydration mismatches.
- Forgetting `enableSystem` on `ThemeProvider`, making system preference detection non-functional.

## Fix Prompt

```txt title="Fix Prompt"
There's a white flash on load or a hydration mismatch. Fix in order:
1. Ensure `ThemeProvider` has `attribute="class"` and `enableSystem` props.
2. Make sure `Providers.tsx` (not `layout.tsx`) is marked `'use client'`.
3. Remove any `theme === 'dark'` conditional rendering from `ThemeToggle` — use `dark:` Tailwind
   variants instead so the server and client render the same HTML.
Show only the corrected diff.
```