---
title: Theme preference
description: Build a complete light, dark, and system theme preference flow.
order: 30
toc:
  - id: define-theme-mode
    title: Define theme mode
  - id: apply-the-theme
    title: Apply the theme
  - id: render-a-control
    title: Render a control
---

## Define theme mode {#define-theme-mode}

```ts
import "@workspace/preferences"

declare module "@workspace/preferences" {
  interface PreferencesCustom {
    "theme-mode": "light" | "dark" | "system"
  }
}
```

Load the cookie on the server and pass the parsed value through `initialPreferences` so server markup and hydration share the same snapshot.

## Apply the theme {#apply-the-theme}

```tsx
const themeEffects = [
  {
    layoutEffect: ({ preferences }) => {
      const systemDark = window.matchMedia(
        "(prefers-color-scheme: dark)"
      ).matches
      const dark =
        preferences["theme-mode"] === "dark" ||
        (preferences["theme-mode"] === "system" && systemDark)

      document.documentElement.classList.toggle("dark", dark)
      document.documentElement.style.colorScheme = dark ? "dark" : "light"
    },
  },
] satisfies readonly PreferenceEffect[]
```

## Render a control {#render-a-control}

```tsx
function ThemeModeSelect() {
  const [themeMode, setThemeMode] = usePreference("theme-mode")

  return (
    <select
      value={themeMode}
      onChange={(event) =>
        setThemeMode(event.currentTarget.value as typeof themeMode)
      }
    >
      <option value="light">Light</option>
      <option value="dark">Dark</option>
      <option value="system">System</option>
    </select>
  )
}
```