Add query_string to all internal links

I have a facebook page tab which iframes my complete blog. When the blog is called by facebook it is done in this way: https://www.myblog.com/?source=facebook.

My templates work this way:

<?php
$source = $_GET[‘source’];
if ($source == “Facebook”) {
?>
// Very minimalistic style, no sidebars, no logo, …
<?php } else { ?>
// Normal Blog Style
<?php } ?>

Thsi works for the first page. When a facebook user follows an internal link of my iframed blog, there is no ?source=facebook and the blogs switches to normal style.

Therefore I need to to add the query_string “?source=facebook” to all internal links (posts, pages, archives, …).

$source = $_GET[‘source’];
if ($source == “Facebook”) { add “?source=facebook” to all internal links }

How can this be done?

Thank you for your help! Peter