Manual:$wgValidSkinNames

Details
List of valid skin names.
 * The key should be the skin's id in all lower case. Generally this should be the same as the name of the base folder inside skins/ that contains the skins css, resources, etc...
 * The value should be an identifier which when prefixed with 'Skin' becomes the class name of the class implemented by your skin. ie: If your skin's class name is 'SkinFooBar' the value in the array should be )
 * 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. ie: 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.

The default skins will be added later, by Skin::getSkinNames. Use Skin::getSkinNames as an accessor if you wish to have access to the full list.