use-theme.tsx

"use client";

import { createContext, useContext, useEffect, useState } from "react";

type Theme = "dark" | "light" | "system";

const ThemeContext = createContext<{ theme: Theme; setTheme: (t: Theme) => void }>({
  theme: "system",
  setTheme: () => {},
});

export function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setTheme] = useState<Theme>("system");

  useEffect(() => {
    const root = document.documentElement;
    root.classList.remove("light", "dark");

    if (theme === "system") {
      const mq = window.matchMedia("(prefers-color-scheme: dark)");
      const apply = () => {
        root.classList.remove("light", "dark");
        root.classList.add(mq.matches ? "dark" : "light");
      };
      apply();
      mq.addEventListener("change", apply);
      return () => mq.removeEventListener("change", apply);
    }

    root.classList.add(theme);
  }, [theme]);

  return (
    <ThemeContext.Provider value={{ theme, setTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

export const useTheme = () => useContext(ThemeContext);