mastodon/app/javascript/flavours/glitch/utils/theme.ts

export function getUserTheme() {
  const { userTheme } = document.documentElement.dataset;
  return userTheme;
}

export function isDarkMode() {
  const { userTheme } = document.documentElement.dataset;
  return (
    (userTheme === 'system' &&
      window.matchMedia('(prefers-color-scheme: dark)').matches) ||
    userTheme !== 'mastodon-light'
  );
}