Using a Child Theme is highly recommended when making ANY changes to your Theme design, including CSS changes. Your Child Theme will be identical to the Responsive Pro Theme and give you the ability to modify everything, while protecting and safe-keeping your changes from future updates.
In essence a WordPress Child Theme is a Theme that inherits the functionality of the parent Theme, and allows you to modify, or add to the functionality of parent theme.
To give you a head start we’ve created a working Responsive Child Theme example for you. Simply download the responsive-child-theme.zip and modify the style.css as you like.
Your Child Theme needs to be either uploaded via FTP or via the WordPress dashboard using the theme upload. Once uploaded simply activate it and that’s it. When Responsive Pro Theme is upgraded your Child Theme and all of your modifications will be safe and remain unchanged.
For more information please have a look at the Child Themes Documentation from the WordPress Codex.