Assign a class to an element dependent on multisite domain?
Well i decided to use different .css files with this code:

// Allow a site-specific user defined CSS file (useful for multisite installations):
// If a CSS file "local-[SITE].css" is residing in the "css" directory (beside "local.css"),
// it will be loaded after "local.css". SITE is the site's host name, without leading "www".
// For example, for the site http://www.mydomain.tld/ the file must be called called "local-[mydomain.tld].css"
global $base_url;
$site = preg_replace("/^[^\/]+[\/]+/", '', $base_url);
$site = preg_replace("/[\/].+/", '', $site);
$site = preg_replace("/^www[^.]*[.]/", '', $site);
drupal_add_css(path_to_theme() . '/css/local-[' . $site . '].css', 'theme', 'all');