How to better distinguish answers from questions when using the Answers module?
Depending on your configuration, there are probably already classes supplied by default with Drupal.  Have you used Firebug to check the markup provided by Drupal by default?  (video tutorial on how to use Firebug)

If you do need to add a class, one quick way to add a class to a field is the Field Group module.  Navigate to the "Manage fields" page for the content type you wish to modify, create a new field group of type div, and then add the class and place the fields you wish to be "in" the class within it.