Child themes are used to make modifications to a theme you like with out messing up the original files. Also, if the theme is updated, you will not loose your changes.
Under wp-content/themes, make a folder for your child theme. Any name will work. Copy the style.css from the theme that you want to modify. At the top of the css change:
/*
Theme Name: Twenty Ten
Theme URI: http://wordpress.org/
Description: The 2010 theme for WordPress is stylish, customizable, simple, and readable — make it yours with a custom menu, header image, and background. Twenty Ten supports six widgetized areas (two in the sidebar, four in the footer) and featured images (thumbnails for gallery posts and custom header images for posts and pages). It includes stylesheets for print and the admin Visual Editor, special styles for posts in the “Asides” and “Gallery” categories, and has an optional one-column page template that removes the sidebar.
Author: the WordPress team
Version: 1.2
License: GNU General Public License
License URI: license.txt
Tags: black, blue, white, two-columns, fixed-width, custom-header, custom-background, threaded-comments, sticky-post, translation-ready, microformats, rtl-language-support, editor-style, custom-menu
*/
To:
/*
Theme Name: Lukeup Child
Theme URI: http: //www.lukeup.com
Description: Child theme for the Twenty Ten theme
Author: Luke Lukas
Author URI: http://www.lukeup.com/about/
Template: twentyten
Version: 0.1.0
*/
Change the info to your info of course.
Add the following line Under the description:
@import url("../twentyten/style.css");
That’s really all there is to it. You can copy the parent themes images folder to your child themes folder in case you want to modify images.
Good Luck.