Download the skin you want (or develop it on your own). Upload it to the skins directory in your Roundcube install. Then in config/main.inc.php find the item that says:
$rcmail_config['skin_path'] = 'skins/default/';
And where the word "default" is, just replace it with the name of the directory of your new skin.
For example, if I downloaded the iTunes skin (available here:
http://roundcubeforum.net/forum/index.php?topic=1795.0 ), I would first unzip it to my desktop. Then, I would upload that folder (named "aqua") to my skins directory in RoundCube. Now, once everything is uploaded, I'd go into config/main.inc.php and edit the line that says $rcmail_config['skin_path'] = 'skins/default/';
to be:
$rcmail_config['skin_path'] = 'skins/aqua/';
Hope that helps.
Also, Till and everyone, please note that a patch was submitted to the forum to choose your skin. You can see it here:
http://roundcubeforum.net/forum/index.php?topic=1651.0
~Brett
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~ Brett Patterson