Once you are familiar with a good CMS and common coding concepts, switching from developing in one CMS to another is fairly straight forward as the process for things tends to be similar. For instance, creating a Drupal sub-theme is conceptually the same as a creating WordPress child theme. You need a file that defines the theme, and what the base (parent) is. To make changes to the theme you add files to override the parent files.
Creating Drupal sub-theme is quite simple. The documentation page “Creating a sub-theme” at http://drupal.org/node/225125 outlines what is needed.
One thing I really like about Drupal sub-themes as compared to WordPress child themes is that Drupal sup-themes have no limit to chaining, you can make sub-themes of sub-themes and so on. WordPress only supports 1 layer of child-theme. This is a issue if you want to make a customized version of a theme that is a child theme of a framework, which is a common configuration for commercial themes.