How do I add custom CSS styles to the body field?
Use the "html inspector" of your browser (Ctr-Shift-I for Chrome) to find the the css classes of thoses fields.

Then add the proper css code in /themes/bartik/css/style.css.

That will be the quickest way to do it but I strongly recommend you to create a sub-theme of the bartik theme (https://drupal.org/node/225125) so if the theme gets updated, you won't loose your changes.

You should check out Display Suite too (https://drupal.org/project/ds), it a fantastique module to change the way nodes are displayed.