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In computing, a skin (also known as visual styles in Windows XP) is a custom graphical appearance achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users. A skin may be associated with themes.
Software that is capable of having a skin applied is referred to as being skinnable, and the process of writing or applying such a skin is known as skinning. Applying a skin changes a piece of software's look and feel – some skins merely make the program more aesthetically pleasing, but others can rearrange elements of the interface, potentially making the program easier to use. Although often used simply as a synonym for skin, the term theme normally refers to less-complex customizations, such as a set of icons and matching colour scheme for an operating system – notably, this is how the term was used in association with Windows 95 and Microsoft Plus! for Windows 95.
Common skinnable applications 
Probably the most popular skins are for instant messaging clients, media center, and media player software, such as Trillian and Winamp, due to the association with fun that such programs try to encourage.
Standard interface 
Of course, it is also possible to change the standard interface. Some platforms have support for this built in, including most using the X Window System. For those that do not, there are usually programs that can add the functionality, like WindowBlinds for Microsoft Windows and ShapeShifter for Mac OS X.
Skinning is typically implemented with a model–view–controller architecture, which allows for a flexible structure in which the interface is independent from and indirectly linked to application functionality, so the GUI can be easily customized. This allows the user to select or design a different skin at will, and also allows for more deep changes in the position and function of the interface elements.
Pros and cons 
The benefit of skinning in user interfaces is disputed. While some find it useful or pleasant to be able to change the appearance of software they use, a changed appearance can complicate technical support and training. A user interface that has been extensively customized by one person may appear completely unfamiliar to another who knows the software under a different appearance. It is often said by usability practitioners that this flexibility "requires the user to be an expert interaction designer" to tailor the software for best use.
Many websites are also skinnable, particularly those which provide some social capabilities. Again, some sites offer skins that make primarily cosmetic changes, while some — such as H2G2 — allow major changes to the layout of pages. As with standalone software interfaces, this is facilitated by the underlying technology of the website — the use of XML and XSLT, for instance, facilitates major changes of layout, while CSS can be used to easily produce different visual styles.