Yeah, that was my though as well.
Only the concept with a plan for all objects required and how they shall look like including the different states for buttons, checkboxes and so on. Maybe different styles. Afterwards the work with Photoshop. Testing. Different versions of programs (e.g. Internet Explorer), unforseen behaviour of WB or SKS....
So it's a very intense hobby if one wants to create really good skins. But it's a very nice job, better than model railways (if this is called like this in English) or such stuff
P.S. OT: sorry, I don't find a setting for notifications of answers/updates in a thread, can somebody tell me where this is located?