app.js

...
// check if the system is currently set to dark, this would return "true" or "false"
const defaultDark = window.matchMedia("(prefers-color-scheme: dark)").matches;

// assuming we are using a state manager as in React.js
// this set the value of "theme" in local storage to "dark" or "light"
const [theme, setTheme] = useLocalStorage(
  "theme",
  defaultDark ? "dark" : "light"
);

// This function is used to handle a button toggle to swap themes
const switchTheme = () => {
  setTheme(theme === "light" ? "dark" : "light");
};