theme-provider.tsxを

tsx
"use client";

import { ThemeProvider as NextThemeProvider } from "next-themes";
import { type ThemeProviderProps } from "next-themes/dist/types";
import { useEffect, useState } from "react";

export const ThemeProvider = ({ children, ...props }: ThemeProviderProps) => {
  const [mounted, setMounted] = useState<boolean>(false);
  useEffect(() => {
    setMounted(true);
    return () => setMounted(false);
  }, []);
  return (
    mounted && <NextThemeProvider {...props}>{children}</NextThemeProvider>
  );
};

のようにすればよいらしい．このようにしたら治った．

参考 DarkMode切替時でのHydration errorの対応策