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 Manual:$wgSkipSkin on how to control that).

Note also that upon registration the value of  is copied to the user preferences; thus setting it to another skin it won't have any effect on registered users, even if they never changed their preferences. See Manual:UserOptions.php for how to change that.

Available skins
This is a full list of the skins that have been available within MediaWiki (without the use of additional skin extensions). The version column has not yet been filled in, but this information needs to be added to the page, as not all these skins were introduced at the same time, and some of them are no longer available in a standard installation.

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:

An alternative way of checking which of the above skins are available in your particular version of MediaWiki is to check.