replace a link in footer generated by theme adding the "rel=something" parameter

hello,
i’m trying to replace the link generated by the theme i’m using (responsive theme from cyberchimps.com) on my website https://www.mycambox.it, so that the G+ icon link on footer area becomes
<a href="blablabla" rel="SOMETHING">
instead of only
<a href="blablabla">

Googling around I found that adding this small piece of code to my functions.php

function replace_content($content)
{
        $content = str_replace('<a href="https://plus.google.com/+SablabItInformatica">', '<a href="https://plus.google.com/+SablabItInformatica" rel="publisher">',$content);
        return $content;
}
add_filter('the_content','replace_content');

should do the trick BUT it seems it works only for the the content among the central area (replacing arbitrary text instead of the link such as a name in the main content area works fine).

I think it’s related to the fact that add_filter calls the_content section and all that’s included in the footer is out of that, so can someone suggest me how to fix that or where to look to understand to what the footer area is “linked”?

sorry if my question is really bad explained but i’m not very much familiar with hacking wp actually nor english native speaker.

Thanks in advance for any help.