.mw-ui-vform > div label specifies an explicit dark color. On dark skins this means the contrast becomes low and in the worst case (the skin isn't using an absolute black) becomes illegible.
This case isn't fixed by simply applying an explicit color somewhere so we'll probably have to create a new 'dark' common/skinning module.
Version: 1.24rc
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=70351