Well I've contacted CPanel and they won't release the Cpanel tags. The only way to get started with a theme would be to look into existing ones and create your own. I did one and it took bout a week or two to complete at my own pace - http://www.hostradius.com/cpaneldemo/
i have developed one. which i will release soon for free, or for an extremely small charge for the time put into it.
i do not have an image up yet....but i like it, so do others that have seen it.
basically look at the tags...you can edit an existing skin (recommended as there are 200+ files) or start from scratch. get a HTML editor and edit each page and image. tags are a bit confusing at times but easy enough to get the jist of with trial and error... PHP and other scripts can be used, which gives a great effect...you'll have to wait and see what i have done!
You need a file called api.txt, which lists all of the cpanel tags. I'm not sure where you find it (ask your webhost), or email me for it. It's just a long list of all of the different variables you can use, i.e.: