Template suggestion for 'content' in custom page
You need to copy the file

entity.tpl.php 


from the theme directory of the module : entity.

Paste it into your theme's template directory, and rename it to 

registration.tpl.php


Other theme suggestions are:

registration__TYPE
registration__TYPE__VIEWMODE
registration__ID


If you want to have access to the preprocess function then add this function to your theme's template.php file :

function MYTHEME_preprocess_entity(&amp;$vars) {
  //dpm($vars);
}