An example of how to detect dark theme preferences (prefers-color-scheme) using window.matchMedia

detect-prefers-color-scheme.js

(function() {
  const prefersDark = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
  const setting = localStorage.getItem('color-schema') || 'auto';
  if (setting === 'dark' || (prefersDark && setting !== 'light')) {
    document.documentElement.classList.toggle('dark', true);
  }
})();