by Luke Rumley on WordPress.org

Select your theme under the category editor, and click Update. Simple!
This plugin allows you to assign a theme to a specific category. It is actually @MikeLopez’s code, I just tweaked it to work on WordPress 3+.
This version adds child theme capability, and the ability to assign a theme to a new category as it is created. It also ignores pages and the home page, so the current theme is applied there.