kcg-monitoring/frontend/src/hooks/useTheme.ts

import { useState, useCallback, useEffect } from 'react';

type Theme = 'dark' | 'light';

const STORAGE_KEY = 'kcg:theme';
const DEFAULT_THEME: Theme = 'dark';

function readStoredTheme(): Theme {
  try {
    const stored = localStorage.getItem(STORAGE_KEY);
    if (stored === 'dark' || stored === 'light') return stored;
  } catch {
    // localStorage unavailable
  }
  return DEFAULT_THEME;
}

function applyTheme(theme: Theme) {
  document.documentElement.dataset.theme = theme;
}

export function useTheme() {
  const [theme, setThemeState] = useState<Theme>(() => {
    const t = readStoredTheme();
    applyTheme(t);
    return t;
  });

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

  const setTheme = useCallback((t: Theme) => {
    setThemeState(t);
    try {
      localStorage.setItem(STORAGE_KEY, t);
    } catch {
      // localStorage unavailable
    }
  }, []);

  const toggleTheme = useCallback(() => {
    setTheme(theme === 'dark' ? 'light' : 'dark');
  }, [theme, setTheme]);

  return { theme, setTheme, toggleTheme } as const;
}