Manual:$wgAllowSiteCSSOnRestrictedPages/en

Details
Many wikis use pages like $css to create a custom 'theme' for their wiki. However, due to security concerns discussed in $bug, 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 $preferences 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 MobileFrontend note that $wgMFSiteStylesRenderBlocking also needs to be enabled for this to work.