The way to use theme() function changes again in Drupal 7, While before in Drupal 6 you can use :

   theme('theme_name', $option1, $option2, $option3);

Now in Drupal 7, You will need to group the $option as an array, thus it will become :

   theme('theme_name', array('option1' => $option1, 'option2' => $option2, 'option3' => $option3));

This somehow will get the theme function much more cleaner but in other hand you will need to type more code to get the same function done.