Start by downloading the latest update (1.05). You will see a new folder named _SYSTEM_ , if you just want to use a new skin. Then just replace those four bmp files with your new ones. As for making your own. I suggest checking those four bmp files to see how they work and then locate a template especially for the icons.bmp to locate the icons correctly.
When creating your own, make sure to save them as 8bit/24bit or they won't show up.