In Drupal 7 there are two ways to programmatically set the theme in which a page is displayed:

• Using hook_menu() + 'theme callback' + 'theme arguments'. This is used for pages which are always displayed in the same theme, such a the administration pages.
• Using hook_custom_theme(). This is used to set the theme for individual pages. For example to set a different theme per user.

Tags: 
hook_menu hook_custom_theme Drupal UX