Hello, DREAMSNET.IT an Italian web company, has developed a skin similar to outllook.com microsoft.
(http://www.dreamsnet.it/wp-content/uploads/roundcube-best-skin-outlook.jpg)
On their website you can view the screenshots, a live demo, and a video on youtube that shows how it works.
Very beautiful.
Link: http://www.dreamsnet.it/2013/07/best-roundcube-skin-look-like-outlook-webmail/ (http://www.dreamsnet.it/2013/07/best-roundcube-skin-look-like-outlook-webmail/)
Youtube video: http://www.youtube.com/watch?v=iZBW3kibIMQ (http://www.youtube.com/watch?v=iZBW3kibIMQ)