plano/demos/use_cases/chatgpt-preference-model-selector/public/init-theme.js

// Apply dark mode based on system preference
if (
  localStorage.theme === 'dark' ||
  (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)
) {
  document.documentElement.classList.add('dark');
} else {
  document.documentElement.classList.remove('dark');
}