For most well-maintained skins (Vector, MonoBook, Modern, CologneBlue, etc.), there are messages called <skin>.css (resp. <skin>.js) with almost the same content /* CSS placed here will affect users of the <skinname> skin */ (<skin> is the skin key, e.g. "monobook" or "cologneblue", and <skinname> is the displayed name, e.g. "MonoBook" or "Cologne Blue").

This could probably be factorised with a standard message in MediaWiki core, with a parameter to add the displayed name of the skin. This should be checked from a linguistic point of view if such a generic message would be correct in every language, but possibly this standard message could be /* CSS placed here will affect users of the $1 skin */. Possibly the translators would have to use the parser function grammar. If it is expected there a some languages where it would be untranslatable (at least automatically), the messages <skin>.css and <skin>.js could be marked as optional, else they could be deleted.

This would decrease (a bit) the number of messages, and it will be more clear for users reaching the empty pages MediaWiki:<skin>.css even for less-maintained skins.