1) Do you use the correct machine name for the theme?
2) Are you sure that there are not other modules to override this later?
3) Is the page cached? If so this may not work properly.
Same question and discussion here: https://drupal.stackexchange.com/questions/812/how-do-i-change-a-theme-based-on-the-url
Useful modules:Page Theme, Context, ThemeKey.