Question

Within the context of a module, how can you determine what theme is loaded for the current user?

drupal_get_path
path_to_theme

Neither of these are any good because they seem to only work in the template.php of a theme.

Answer 1

If users are allowed to select a theme for themselves, the theme they selected is saved in $user->theme, where $user is the user object.
The global variable $custom_theme contains the name of the theme currently set, if a module has set a custom theme.

The following snippet saves in $current_theme the name of the theme currently active:

global $custom_theme, $theme, $user;

if (!empty($user->theme)) {
  $current_theme = $user->theme;
}
elseif (!empty($custom_theme)) {
  $current_theme = $custom_theme;
}
else {
  $current_theme = $theme ? $theme : variable_get('theme_default', 'garland');
}