style.css is the one and only required file in a child theme. It provides the information header by which WordPress recognizes the child theme, and it replaces the style.css of the parent.
As with any WordPress theme, the information header must be at the top of the file, the only difference being that in a child theme the Template: line is required, so that WordPress knows which theme is the parent of the child theme.
Here is an example information header of a child theme’s style.css:
Theme Name: Twenty Eleven Child
Theme URI: http: //example.com/
Description: Child theme for the Twenty Eleven theme
Author: Your name here
Author URI: http: //example.com/about/
A quick explanation of each line:
- Theme Name. (required) Child theme name.
- Theme URI. (optional) Child theme webpage.
- Description. (optional) What this theme is. E.g.: My first child theme. Hurrah!
- Author URI. (optional) Author webpage.
- Author. (optional) Author name.
- Template. (required) directory name of parent theme, case-sensitive.
- NOTE. You have to switch to a different theme and back to the child theme when you modify this line.
- Version. (optional) Child theme version. E.g.: 0.1, 1.0, etc.
The part after the closing */ of the header works as a regular stylesheet file. It is where you put the styling rules you want WordPress to apply.
more details: http://codex.wordpress.org/Child_Themes