Modify the Theme after the user login or set user default theme on registration. How to?

Hi,

I am new to Rules.

I would like to change Theme after the user login. In my web site, the user cannot change the theme.

In my mind, I have two choices:

1) Modify the Theme after the user login
2) set user default theme on registration.

Could this be done by Rules? We all know that we have the trigger "after a user login". Therefore, my main question is how to create the Action? Is it a must to write some PHP code?

Thanks.

hosais