Luke Rumley

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.

GYS Themed Categories

This plugin allows you to assign a theme to a specific category.

Masahiro NAKASHIMA

The theme can be changed and displayed to only logged in users.
The visitors of the website see your old theme while you are testing the new one.
Only the administrator of the WordPress site sees the new theme that can now be tested safely.
You can also use this plugin instead of “Theme Test Drive” plugin.

WP Theme Test

I have xdebug installed. There are two big Notices.

Threw two big PHP notices

The theme can be changed and displayed to only logged in users.

GYS Themed Categories vs WP Theme Test