Thread:Project:Support desk/MediaWiki:Vector.css not applied to Special:Preferences/reply

User CSS and user JS isn't allowed on Special:UserLogin and Special:Preferences, so it will not be loaded on both special pages for security reasons (see ). You have to put the CSS to the css files of your skin or load it via LocalSettings.php.

There is a bug report for this, too, with a discussion to re-allow usercss and user js on both special pages:.