Manual:$wgAllowSiteCSSOnRestrictedPages

Details
Many wikis use pages like MediaWiki:Common.css to create a custom 'theme' for their wiki. http://www.cs.berkeley.edu/~daw/teaching/cs276-s06/mtsol.ps

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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.

This makes it so that, by default, such style sheets no longer load on login>Special:UserLogin|login or preferences>Special:Preferences|preference pages, thus creating an inconsistent user interface. =

In MediaWiki 1.19.22, 1.22.14, 1.23.7 and newer,   now allows to re-enable site-wide CSS on restricted special pages.