A Vite plugin that stops @42-central/ui-react apps from flashing the wrong theme on load. It puts the kit's theme-init script at the top of <head> in index.html, in both vite dev and vite build.

Why

useTheme() stores the visitor's "light" | "dark" | "system" choice and keeps data-theme on <html> in sync. React only runs after first paint, though. So without help, a dark-mode visitor sees one light frame before the hook catches up.

In Next.js, <ThemeScript /> covers that gap from the server render. A Vite SPA has no server render, so the script has to sit in index.html and block parsing. It also can't be type="module", because Vite defers module scripts, and the flash comes back. This plugin injects a classic inline <script> through Vite's transformIndexHtml hook, so you never edit the HTML by hand.

Install

Install it as a dev dependency, next to the kit:

npm install -D @42-central/vite-plugin-theme
pnpm add -D @42-central/vite-plugin-theme
yarn add --dev @42-central/vite-plugin-theme
bun add --dev @42-central/vite-plugin-theme

vite 7 or 8 is a peer dependency. The plugin builds its script with @42-central/ui-react, which it lists as its own dependency. Keep both packages on the same version: they are released together.

Usage

// vite.config.ts
import { defineConfig } from "vite";
import react from "@vitejs/plugin-react";
import { themeInitPlugin } from "@42-central/vite-plugin-theme";

export default defineConfig({
  plugins: [react(), themeInitPlugin()],
});

Then read and change the theme from any component with the kit's hook:

import { useTheme } from "@42-central/ui-react";

export function ThemeToggle() {
  const { resolvedTheme, setMode } = useTheme();
  return (
    <button onClick={() => setMode(resolvedTheme === "dark" ? "light" : "dark")}>
      Toggle theme
    </button>
  );
}

Options

themeInitPlugin(options?) takes the same options as the kit's buildThemeInitScript(), plus nonce:

| Option | Type | Default | What it does | | -------------------- | ---------------------------------------------------- | -------------- | ----------------------------------------------------------------------------------- | | storageKey | string | "ui-theme" | localStorage key that holds the stored mode. | | defaultColorScheme | "light" \| "dark" \| "system" | "system" | Mode used when nothing is stored yet. | | forceColorScheme | "light" \| "dark" | – | Pins the theme and ignores both storage and the OS preference. | | attribute | "data-theme" \| "class", or an array of them | "data-theme" | What to write on <html>: the data-theme attribute, a .dark class, or both. | | nonce | string | – | CSP nonce added to the injected <script>. |

Keep it in sync with useTheme()

There is no provider holding this configuration, so the script and the hook can't share it. Pass storageKey, defaultColorScheme, forceColorScheme and attribute with the same values to the plugin and to every useTheme() call. If they differ, the script and the hook disagree on first load.

// vite.config.ts
themeInitPlugin({ forceColorScheme: "dark", attribute: ["class", "data-theme"] });

// anywhere in the app
useTheme({ forceColorScheme: "dark", attribute: ["class", "data-theme"] });

A shared const THEME_OPTIONS = { … } imported by both files is the easiest way to keep them aligned.

Content Security Policy

If your CSP blocks inline scripts, pass the nonce your server puts in the script-src directive:

themeInitPlugin({ nonce: "{{CSP_NONCE}}" });

The value goes into the built index.html verbatim. Use whatever placeholder your server or edge function replaces per request.

What it injects

One tag, first in <head>:

<head>
  <script>/* ~600 bytes: read localStorage, resolve "system", set data-theme */</script>
  <!-- the rest of your head -->
</head>

The script runs once and registers no listeners. Following the OS preference live after load is useTheme()'s job.

Not using Vite?

The ui-react README covers the alternatives: a prebuilt theme-init.js you copy into public/, or pasting the output of buildThemeInitScript() by hand. Next.js apps use <ThemeScript /> instead.

Links

• Theming docs
• @42-central/ui-react
• Source on GitHub