To do so You should at first build an advanced theme , please see here:
http://www.question2answer.org/advanced.php
You basically copy some or all the html output functions to a new file and then tell q2a to use that new file for html output. It is very easy. Once ready You should look out for the matching function, which should be
function q_view_main($q_view)
which includes following lines or similar:
$this->q_view_content($q_view); <<<<this line outputs the content of question
$this->post_tags($q_view, 'qa-q-view'); <<<<this line outputs the tags
between these lines You would add:
$this->output('<hr>');
(I changed my files a lot so it may be slightly different in Your case)
On the other hand You can achieve the same by just adding some padding or margin in the css file.
monk333