Editing of sitewide CSS/JS is only possible for interface administrators from nowSunting
(Please help translate to your language)
as announced previously, permission handling for CSS/JS pages has changed: only members of the
interface-admin (Pengurus antarmuka) group, and a few highly privileged global groups such as stewards, can edit CSS/JS pages that they do not own (that is, any page ending with .css or .js that is either in the MediaWiki: namespace or is another user's user subpage). This is done to improve the security of readers and editors of Wikimedia projects. More information is available at Creation of separate user group for editing sitewide CSS/JS. If you encounter any unexpected problems, please contact me or file a bug.