You can add a MediaQueryListListener by calling addListener() on the MediaQueryList returned by Window.matchMedia():

function activateDarkMode() {
  // set style to dark
}

window.matchMedia("(prefers-color-scheme: dark)").addListener(
  e => e.matches && activateDarkMode() // listener
);

See this How-To article on Use “prefers-color-scheme” to detect macOS dark mode with CSS and Javascript

Newer version to register the listener

Above method addListener(listener) has become deprecated but still works as an alias for newer addEventListener("change", listener)). (Thanks to lukaszpolowczyk for pointing out!)

With initialization setter and 'change' listener.

const colorSchemeQueryList = window.matchMedia('(prefers-color-scheme: dark)');
  
const setColorScheme = e => {
  if (e.matches) {
    // Dark
    console.log('Dark mode')
  } else {
    // Light
    console.log('Light mode')
  }
}
  
setColorScheme(colorSchemeQueryList);
colorSchemeQueryList.addEventListener('change', setColorScheme);