Manual:$wgAllowSiteCSSOnRestrictedPages

Details
Due to security considerations as described with bug 70672 side-wide CSS via e.g. MediaWiki:Common.css was disabled starting with MediaWiki 1.19.20, 1.22.12 and 1.23.5. However many wikis use this and associated pages to create a custom "theme" for their wiki, which would no longer load on login or preference pages, thus creating an inconsistent user interface (see bug 71621 for further information).

This configuration setting allows to re-enable side-wide CSS on restricted special pages which is by default disabled to be secure.