React Hook recipe from https://usehooks.com. Demo: https://codesandbox.io/s/p33j1m0mxj

use-dark-mode.jsx

// Usage
function App() {
  const [darkMode, setDarkMode] = useDarkMode();

  return (
    <div>
      <div className="navbar">
        <Toggle darkMode={darkMode} setDarkMode={setDarkMode} />
      </div>
      <Content />
    </div>
  );
}

// Hook
function useDarkMode() {
  // Use our useLocalStorage hook to persist state through a page refresh.
  // Read the recipe for this hook to learn more: usehooks.com/useLocalStorage
  const [enabledState, setEnabledState] = useLocalStorage('dark-mode-enabled');

  // See if user has set a browser or OS preference for dark mode.
  // The usePrefersDarkMode hook composes a useMedia hook (see code below).
  const prefersDarkMode = usePrefersDarkMode();

  // If enabledState is defined use it, otherwise fallback to prefersDarkMode.
  // This allows user to override OS level setting on our website.
  const enabled =
    typeof enabledState !== 'undefined' ? enabledState : prefersDarkMode;

  // Fire off effect that add/removes dark mode class
  useEffect(
    () => {
      const className = 'dark-mode';
      const element = window.document.body;
      if (enabled) {
        element.classList.add(className);
      } else {
        element.classList.remove(className);
      }
    },
    [enabled] // Only re-call effect when value changes
  );

  // Return enabled state and setter
  return [enabled, setEnabledState];
}

// Compose our useMedia hook to detect dark mode preference.
// The API for useMedia looks a bit weird, but that's because ...
// ... it was designed to support multiple media queries and return values.
// Thanks to hook composition we can hide away that extra complexity!
// Read the recipe for useMedia to learn more: usehooks.com/useMedia
function usePrefersDarkMode() {
  return useMedia(['(prefers-color-scheme: dark)'], [true], false);
}

This code:

if (enabled) {
  element.classList.add(className);
} else {
  element.classList.remove(className);
}

can be simplified to:

element.classList.toggle(className, enabled);