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next-themes injects its script inside the React tree, which React 19 reports: put a small boot script in head

react@19.3

I assumed: next-themes was the standard way to give a Next.js app light, dark and system themes without a flash.
What happened: it renders its no-flash script inside the React component tree, and React 19 reports a script rendered there as an error.
The fix: a 60-line theme store, and a small inline script that the root layout puts in <head>. Before the first paint it applies the saved theme and the customiser's choices (colour preset, radius, density, font, motion, sidebar style, width) to <html>.
Evidence: docs/DECISIONS.md item 12; themeBootScript in src/store/ui.ts; src/app/[locale]/layout.tsx lines 46–48.

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next-themes injects its script inside the React tree, which React 19 reports: put a small boot script in head

I assumed: next-themes was the standard way to give a Next.js app light, dark and system themes without a flash.
What happened: it renders its no-flash script inside the React component tree, and React 19 reports a script rendered there as an error.
The fix: a 60-line theme store, and a small inline script that the root layout puts in <head>. Before the first paint it applies the saved theme and the customiser's choices (colour preset, radius, density, font, motion, sidebar style, width) to <html>.
Evidence: docs/DECISIONS.md item 12; themeBootScript in src/store/ui.ts; src/app/[locale]/layout.tsx lines 46–48.

Learned in the articleA restaurant POS with no backend: Sofreh's cross-tab sync, a seed in a Web Worker and seven traps

Something wrong in it, or against the rules? Report it.