margin / web / src / context / ThemeContext.jsx

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

const ThemeContext = createContext({
  theme: "system",
  setTheme: () => null,
});

export function ThemeProvider({ children }) {
  const [theme, setTheme] = useState(() => {
    return localStorage.getItem("theme") || "system";
  });

  useEffect(() => {
    localStorage.setItem("theme", theme);

    const root = window.document.documentElement;
    root.classList.remove("light", "dark");

    delete root.dataset.theme;

    if (theme === "system") {
      const systemTheme = window.matchMedia("(prefers-color-scheme: dark)")
        .matches
        ? "dark"
        : "light";

      if (systemTheme === "light") {
        root.dataset.theme = "light";
      } else {
        root.dataset.theme = "dark";
      }
      return;
    }

    if (theme === "light") {
      root.dataset.theme = "light";
    }
  }, [theme]);

  useEffect(() => {
    if (theme !== "system") return;

    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
    const handleChange = () => {
      const root = window.document.documentElement;
      if (mediaQuery.matches) {
        delete root.dataset.theme;
      } else {
        root.dataset.theme = "light";
      }
    };

    mediaQuery.addEventListener("change", handleChange);
    return () => mediaQuery.removeEventListener("change", handleChange);
  }, [theme]);

  const value = {
    theme,
    setTheme: (newTheme) => {
      setTheme(newTheme);
    },
  };

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

// eslint-disable-next-line react-refresh/only-export-components
export function useTheme() {
  const context = useContext(ThemeContext);
  if (context === undefined)
    throw new Error("useTheme must be used within a ThemeProvider");
  return context;
}