Responsive ii (codenamed “Responsive Mobile”) is a popular theme on WordPress.org with over 6000 active installs and is updated regularly with new features.
When making ANY changes to your theme design, including CSS changes, it is recommended that you use a Child Theme.
If you overwrite the theme files directly without creating a child theme, it will be hard for you to update your theme every time without losing your changes.
Your child theme will be identical to the Responsive II theme and give you the ability to modify everything, while protecting and safe-keeping your changes from future updates.
So, basically, 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 help you get started, here is a working Responsive II child theme example for you. You can go ahead and download the Responsive II child theme 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 II is upgraded your child theme and all of your modifications will be safe and remain unchanged.
For more help with Child Themes, you could check out child themes documentation on the WordPress Codex.