Yes it is possible.
./config/main.inc.php
// skin name: folder from skins/
// map your skins here 'domain/host' => 'skin_name'
$skins = array(
'mail.roundcube.de' => 'default',
'roundcube.de' => 'groupvice4',
);
$pageURL = 'http';
if ($_SERVER['HTTPS'] == 'on')
$pageURL .= 's';
$pageURL .= '://';
if ($_SERVER['SERVER_PORT'] != '80')
$pageURL .= $_SERVER['SERVER_NAME'].':'.$_SERVER['SERVER_PORT'].$_SERVER['REQUEST_URI'];
else
$pageURL .= $_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI'];
$result = parse_url($pageURL);
if(get_input_value('skin', RCUBE_INPUT_GPC))
$rcmail_config['skin'] = get_input_value('skin', RCUBE_INPUT_GPC);
elseif(isset($skins[$result['host']]))
$rcmail_config['skin'] = $skins[$result['host']];
else
$rcmail_config['skin'] = 'default';