frontend/src/lib/theme.ts

export const THEME_STORAGE_KEY = 'nexus-theme';

export type ThemePreference = 'light' | 'dark' | 'system';

export type ResolvedTheme = 'light' | 'dark';

export function getSystemTheme(): ResolvedTheme {
  if (typeof window === 'undefined') return 'dark';
  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
}

export function resolveTheme(preference: ThemePreference): ResolvedTheme {
  if (preference === 'system') return getSystemTheme();
  return preference;
}

export function applyTheme(preference: ThemePreference) {
  const resolved = resolveTheme(preference);
  const root = document.documentElement;
  root.classList.toggle('dark', resolved === 'dark');
  root.dataset.theme = resolved;
  root.style.colorScheme = resolved;
}

export function readStoredTheme(): ThemePreference {
  if (typeof window === 'undefined') return 'dark';
  const stored = window.localStorage.getItem(THEME_STORAGE_KEY);
  if (stored === 'light' || stored === 'dark' || stored === 'system') return stored;
  return 'dark';
}