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.
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.