Manual:$wgAllowSiteCSSOnRestrictedPages

Details
This configuration setting allows wikis to re-enable site-wide CSS on restricted special pages, which is (as of   MediaWiki versions 1.19.20, 1.22.12, and 1.23.5) disabled by default due to security concerns discussed in bug 70672, site-wide CSS was disabled starting with MediaWiki versions 1.19.20, 1.22.12 and 1.23.5. Many wikis use pages like   MediaWiki:Common.css to create a custom 'theme' for their wiki, but the fact that this change has been implemented makes it so that, by default, such style sheets no longer load on login or preference pages, thus creating an inconsistent user interface.