There is no other way to change the theme automatically of a WordPress blog manually or by any other coding. The only option is to do it by this WordPress plugin. If you wish to change the theme of your WordPress blog automatically then you can use this WordPress plugin. You can specify the conditions in the plugin settings page when you want the theme to be changed automatically.
Here are the features of this plugin:
1) Change WordPress theme based on conditions: By this plugin you can automatically change the entire theme for single posts, authors, categories, tags, custom fields, archive, search, custom posts, custom taxonomies terms, logged in user, user roles,weeks, days and months. You can set the themes which needs to be changed depending on the conditions in the admin panel.
2) Drag and drop interface: Themes can be changed to a particular condition by using drag and drop interface in the plugin settings page.
3) Unlimited conditions: You also have the option to select multiple conditions for a theme to be switched in the front end.