Hoe de onChange-gebeurtenis voor de drop-down user-fields op de aanmeldingspagina aan te passen

Hi All,
I have added new fields “Country” and “State” on Sign-up page using User Fields functionality.
Now I want to display/hide State filed based on Selected Country. How to achieve this using Theme component.

In other words I want to control onChange event for “Country” drop-down field.

Thank you.