Manual:$wgValidSkinNames

Details
Array with information on valid skin names. Optionally can be used by skins to tell MediaWiki that they are available.

Generally this should be the same as the name of the base folder inside skins/ that contains the skins css, resources, etc...
 * The key should be the skin's id in all lower case.

i.e. If your skin's class name is "SkinFooBar" the value in the array should be  
 * The value should be an identifier which when prefixed with "Skin" becomes the class name of the class implemented by your skin.

i.e. For   it would try to load the "SkinFoobar" class. This caused issues with our AutoLoader being case-sensitive despite PHP class names being case-insensitive.
 * Prior to MediaWiki 1.18 the value was a fallback display name and the class name was determined by running ucfirst on the lowercase skin id.

The default skins will be added later, by  .

Use   as an accessor if you wish to have access to the full list.