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).

これは新規利用者と匿名閲覧者のために既定外装を指定しただけです. 匿名閲覧者と登録利用者で外装の変更を個人設定ページに指定していない、もしくは一時的に外装を無効にしている場合、サイトはこの外装を表示します. (利用者が個人設定で選ぶ外装を制限するには を使用します. )

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 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.

Legacy information
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: