A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme. You only need it if you want to introduce a change that is impossible by using the Theme options panel. Please note that when you activate a child theme the menus, sidebars and widgets dis-attached and you will have to attach them again.
Why use a Child Theme?
There are a few reasons why you would want to use a child theme:
- If you modify a theme directly and it is updated, then your modifications may be lost. By using a child theme you will ensure that your modifications are preserved.
- Using a child theme can speed up development time.
- Using a child theme is a great way to to learn about WordPress theme development.
Please do not try to set up a child theme if you are not developer.
You can find sample child theme here: http://vamtam.com/child-themes/
How to add custom CSS code modify the theme code ?
The official WordPress documentation explains this topic really well: http://codex.wordpress.org/Child_Themes