suppress React 19 script warning by not rendering ThemeScript on client

[jakubwarkusz]

Problem

Since Next.js 16.2 ThemeProvider triggers a browser warning:

Encountered a script tag while rendering React component. Scripts inside React components are never executed when rendering on the client. Consider using template tag instead

Reported in #385.

Fix

Return null from ThemeScript when running on the client
(typeof window !== 'undefined'). The script only needs to be present in the
SSR-generated HTML where the browser executes it once before hydration. After
that it is no longer needed.

The existing nonce prop, which was already gated behind
typeof window === 'undefined', is simplified accordingly.

[jakubwarkusz]

This is also fixed by my own library themes.wrksz.dev that's a near drop-in replacement for next-themes. See this

[bolechen]

+1 — we ran into the same Encountered a script tag while rendering React component. Scripts inside React components are never executed warning on React 19 / Next.js 16.2, most reliably on localized 404 renders where ThemeProvider re-mounts and ThemeScript ends up running on the client.

We shipped an equivalent fix via pnpm patch — adding if (typeof window !== 'undefined') return null at the top of ThemeScript, leaving the SSR path untouched so the FOUC-prevention script is still injected — and added e2e coverage across locales × light/dark/system on direct 404 visits plus client-side navigation, asserting no script warning and correct theme init. All green, no regression, warning gone.

This looks like the correct minimal fix. Is there anything blocking a merge here? CI is passing — happy to help if a rebase or test additions would help it land.