I have quickly coded a skin for calendar plugin. The only remaining issue is that the background color in the main navigation bar is not set active/passive.
Maybe the devs of the skin will fix it ...
EDIT:
jqueryui plugin:
Map it to redmond theme:
config.inc.php
// map Roundcube skins with jquery-ui themes here
$rcmail_config['jquery_ui_skin_map'] = array(
'groupvice4' => 'redmond',
'meh' => 'redmond',
'litecube' => 'redmond',
);