After a couple of hours of tries, I understood how to add a roundcube skin on my server.
Let me share with you how to do it.

 MY SERVER

• Ubuntu Server 14.04
• ISPConfig 3.0.5.4
• RoundCube webmail 0.9.5

ROUNDCUBE INSTALLATION STEPS

I used a tutorial found on "how to forge", see references.

That tutorial is for debian but it worked fine on my ubuntu server.

Just a little difference between debian and ubuntu; when the tutorial asked to edit /etc/apache2/conf.d/roundcube, you have to edit /etc/roundcube/apache.conf

ADDING A NEW ROUNDCUBE SKIN ON AN ISPCONFIG ENVIROMENT

1. Download your fav skin, I downloaded the googie-larry skin, I like it, it has some fixes plugins for ispconfig for a better integration.
2. Upload the folder skin in the roundcube skins folder
   in my case the googie-lanny was uploaded in /usr/share/roundcube/skins
3. If your fav skin provide the ispconfig integration plugins (googie-larry provides them), download it and upload the plugin folders in the roundcube plugins folders.
   In my case I uploaded in /var/lib/roundcube/plugins
4. Now, last step where I loose a couple of hours.
   In order to see the new skin in Settings->Preferences->User Interface->interface Skin, you have to create a symbolink links

THE SYMBOLIC LINK

The skin folder /usr/share/roundcube/skins contains the googie_larry uploaded
root@host7:~# ls /usr/share/roundcube/skins
drwxr-xr-x 7 root root 4096 Dec  1 20:32 .
drwxr-xr-x 5 root root 4096 Nov 25 00:33 ..
drwxr-xr-x 5 root root 4096 Nov 25 01:00 classic
drwxr-xr-x 6 root root 4096 Dec 1  06:45 googie_larry
drwxr-xr-x 5 root root 4096 Nov 25 00:33 larry

As you can see the folder /var/lib/roundcube/skins has not the symbolic link for the skin googie_larry.

The symbolic links in /var/lib/roundcube/skins seems a must if you want to see the skin in the roundcube setting.

root@host7:~# ls /var/lib/roundcube/skins
lrwxrwxrwx 1 root root 34 Jan 21 2014 classic -> /usr/share/roundcube/skins/classic
lrwxrwxrwx 1 root root 32 Jan 21 2014 larry -> /usr/share/roundcube/skins/larry

 Create the symbolic links 

root@host7:~# ln -s /usr/share/roundcube/skins/googie_larry /var/lib/roundcube/skins/googie_larry
root@host7:~# ls /var/lib/roundcube/skins
drwxr-xr-x 2 root root 4096 Dec  1 06:47 .
drwxr-xr-x 6 root root 4096 Nov 25 00:33 ..
lrwxrwxrwx 1 root root 34 Jan 21 2014 classic -> /usr/share/roundcube/skins/classic
lrwxrwxrwx 1 root root 39 Dec 1 06:47 googie_larry -> /usr/share/roundcube/skins/googie_larry
lrwxrwxrwx 1 root root 32 Jan 21 2014 larry -> /usr/share/roundcube/skins/larry

FINALLY

Here what I see in my roundcube at Settings->Preferences->User Interface->interface Skin

- have fun -