The ‘Preferences’ and ‘Login’ pages on 1.23.5 are not affected by any changes I make to Common.css and Vector.css. I didn’t have this problem on 1.22.
Others have tested it and report the same result. Is there any solution to this?
Topic on Talk:MediaWiki 1.23
Currently not except for creating a custom skin or adapting the standard skins at server level (be cautious when upgrading in the latter case). This behaviour was introduced with 1.19.20, 1.22.12 and 1.13.5 so solve bugzilla:70627. Bug bugzilla:71621 was opened to address your very issue (you are not alone) but this is currently getting anywhere as you can see. Still searching for solutions here.
See also Thread:Project:Support desk/How to add custom CSS via LocalSettings.php (2). Note that this require someone to edit the CSS and JS for the site on files on the server, rather than just editing MediaWiki pages.