Manual:$wgDefaultSkin

Details
This setting defines the default skin and should be set to the lowercase name of the skin (e.g. 'monobook', not 'MonoBook').

Note that this only sets the default skin, for new users and anonymous visitors. Registered users may change this to any one of the other available skins through their preferences page (see skipskins>Special:MyLanguage/Manual:$wgSkipSkins|Manual:$wgSkipSkins on how to control that).

Available skins
Several skins are included with the MediaWiki installer tarball (the table below details the versions they were/are available with). You can find more skins in cat-skins>:Category:All skins|Category:All skins.

The first column gives the ID which should be used to configure this setting, the second gives the name as it appears within the MediaWiki interface and the third indicates the versions of MediaWiki that this skin is available for: