fabro/apps/arc-web/app/lib/theme.tsx

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

type Theme = "light" | "dark";

const STORAGE_KEY = "arc-theme";

function getInitialTheme(): Theme {
  if (typeof window === "undefined") return "dark";
  const stored = localStorage.getItem(STORAGE_KEY);
  if (stored === "light" || stored === "dark") return stored;
  return window.matchMedia("(prefers-color-scheme: dark)").matches
    ? "dark"
    : "light";
}

function applyThemeClass(theme: Theme) {
  const root = document.documentElement;
  root.classList.remove("light", "dark");
  root.classList.add(theme);
}

const ThemeContext = createContext<{
  theme: Theme;
  toggle: () => void;
}>({ theme: "dark", toggle: () => {} });

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

  useEffect(() => {
    applyThemeClass(theme);
  }, [theme]);

  const toggle = useCallback(() => {
    setTheme((prev) => {
      const next = prev === "dark" ? "light" : "dark";
      localStorage.setItem(STORAGE_KEY, next);
      return next;
    });
  }, []);

  return (
    <ThemeContext value={{ theme, toggle }}>
      {children}
    </ThemeContext>
  );
}

export function useTheme() {
  return useContext(ThemeContext);
}

export type { Theme };