Here are my notes on WordPress Child Themes. I made one for zbench and am now working with Nulis. The key wordpress codex page is here…. It documents how to create a child theme. The technique for defining inheritance has changed between the time when I made for the zbench child theme and today, when I first started to try to customise nulis. The new technique requires a function.php file in the child theme folder which has code to control the order in which the parent and child files, including the CSS are loaded.
I ran a google query with the keywords “wordpress child themes”.
Hongkiat have an article on developing with child themes.
I returned to this in Feb ’02 when I needed to perform significant surgery on the simple theme. Here are some links I found useful.