Manual:$wgAllowSiteCSSOnRestrictedPages

Details
Many wikis use pages like MediaWiki:Common.css to create a custom 'theme' for their wiki. However, due to security concerns discussed in, site-wide CSS on restricted special pages was disabled starting with MediaWiki versions 1.19.20, 1.22.12 and 1.23.5. By default such style sheets no longer load on login or preference pages creating an inconsistent user interface. In MediaWiki 1.19.22, 1.22.14, 1.23.7 and newer, this configuration parameter allows to re-enable site-wide CSS on restricted special pages.

If using the MobileFrontend extension note that  also needs to be enabled for this to work.