In computing, skins and themes are custom graphical appearances (GUIs) that can be applied to certain software and websites in order to suit the different tastes of different users. Such software 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 customisations, 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.

Probably the most popular customised skins are for instant messengers and media players, such as the AOL Instant Messenger, and Winamp, due to the association with fun such programs try to encourage. However, in programs which use a cross-platform GUI toolkit, rather than using the standard interface for the operating system on which they are running, it is relatively easy to add functionality to alter this interface from within the program. For instance, both the Mozilla and Opera web browsers are skinnable because they take advantage of a cross-platform toolkit. In the case of Mozilla, the entire interface is written in XUL, CSS, and JavaScript. Those who know these languages are free to modify both its look and its behavior.

Of course, it is also possible to change the standard interface. Some platforms have inbuilt support for this, including most using the X Window System. For those that do not, there are usually programs that can add this functionality, like WindowBlinds for Microsoft Windows and Kaleidoscope for Mac OS.

Many websites are also skinnable, particularly those which provide some interactive 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.

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 totally unfamiliar to another person who knows the same software under a different appearance.