Add a class to the  tag on a node view
This is quite simple and could likely be done in template.php. I would start with the following:


hook_preprocess_html() to hook into the rendering of the &lt;body&gt; tag.
menu_get_object() for loading the current node (in some themes it might already be present in $variables as  $variables['node']).
Grab the data the user entered into the field using field_get_items or something similar.
Pass it to drupal_clean_css_identifier to ensure it's ready to be used as a class.
Add it to the end of the $variables['classes_array'] variable.
Clear the cache to ensure your new hook is picked up.