teams.spfx.css.ts

const [styleState, setStyleState] = useState<string>(styles.containerdefault);

//Run anytime the themeState changes. 
useEffect(() => {

  console.log("themeState useEffect fired");

  switch (themeState) {
    case "dark":
      setStyleState(styles.containerdark);
      break;
    default:
      setStyleState(styles.containerdefault);
  }

}, [themeState]);