Apart from all the long-known meanings of skin, the computer term means a customised user graphic interface for an operating system - in other words, the outer appearance of a website or software. A basic operational system can appear quite different by applying a new skin. This skinning process is done either to make the look more pleasing or to make it easier to use.
In Old English, the word WITH meant 'against'. This meaning is still preserved in phrases such as 'to fight with'