Is there any way to see the content of the theme registry?
If you want to see the content of the theme registry, you can use _theme_load_registry(). The first argument could be the global $theme for the currently used theme, or one of the objects returned by list_themes().
As the function name is prefixed by an underscore, I would suggest to use it only for debugging purposes. (Theoretically, future Drupal versions could drop those private functions, and that would not probably be taken as API change.)

If you have installed Devel, you can use the following code:

global $theme;

$registry = _theme_load_registry($theme);
dsm($registry);


You can also simply visit the devel/theme/registry page which shows you the theme registry content.