agent-evals/frontend/src/hooks/useTheme.ts

import { useState, useEffect } from 'react';
import type { Theme } from '../types';

const THEME_KEY = 'markdown-editor-theme';

export function useTheme() {
  const [theme, setThemeState] = useState<Theme>(() => {
    const stored = localStorage.getItem(THEME_KEY);
    if (stored && ['light', 'dark', 'system'].includes(stored)) {
      return stored as Theme;
    }
    return 'system';
  });
  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');

  useEffect(() => {
    const root = window.document.documentElement;

    const updateTheme = () => {
      let resolved: 'light' | 'dark';
      if (theme === 'system') {
        resolved = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
      } else {
        resolved = theme;
      }
      resolvedTheme !== resolved && setResolvedTheme(resolved);

      if (resolved === 'dark') {
        root.classList.add('dark');
      } else {
        root.classList.remove('dark');
      }
    };

    updateTheme();

    if (theme === 'system') {
      const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
      const handler = () => updateTheme();
      mediaQuery.addEventListener('change', handler);
      return () => mediaQuery.removeEventListener('change', handler);
    }
  }, [theme]);

  const setTheme = (newTheme: Theme) => {
    setThemeState(newTheme);
    localStorage.setItem(THEME_KEY, newTheme);
  };

  return { theme, resolvedTheme, setTheme };
}