Sorry, Christophdollis,
I was very selfish not sharing the solution I found. But, please don’t get me wrong. Since I found the solution on this very same forum, I thought that others could also find it easily, and that I was just being a bit slow.
Anyway, here it goes the solution I found. It was very very simple, you just need to put the alternate stylesheets in your theme’s folder and add the template tag <?php bloginfo('template_directory'); ?>
to the path to it. The code will look like this in the header of your theme:
<link href="<?php bloginfo('template_directory'); ?>/youralternatestylesheet.css" rel="alternate stylesheet" type="text/css" title="youralternatestylesheettitle" />