Manual:$wgDefaultSkin/ja

詳細
この設定は既定の外装 (スキン) を設定します. 外装名は小文字で指定しなければなりません (例えば、'MonoBook' ではなく 'monobook' のように).

Note that this only sets the default skin. The site renders in this skin to anonymous visitors and to registered users who have not chosen a different skin in Preferences or temporarily overridden the skin. (Use to limit the skins that users can choose in Preferences.)

Changing user preferences when removing a skin
In versions up to at most 1.11, upon registration the value of  was copied to the user preferences; thus setting $wgDefaultSkin to another skin would not have had any effect on registered users, even if they never changed their preferences. And it did not have an effect on registered users, who did change the skin to use.

When you remove a skin from the list of skins to choose from, use the maintenance script Manual:UserOptions.php to make sure that this skin is also not chosen by any user:

This will change the skin for all registered users, who have selected Monobook, to Vector.

利用できる外装
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 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: