Just found this out and thought it might be useful to someone else.

If you’re running KDE and using the GTK-QT Engine to run GTK applications (e.g. Firefox) with your native KDE styles like I am, you may notice that after upgrading your software or changing your KDE colour scheme that certain bits of GTK apps suddenly turn grey.

Well, easy solution.

K Menu > Control Centre (sometimes also Personal Settings or something similar) > Appearance and Themes > Colours. Then make sure ‘Apply colours to non-KDE applications‘ is ticked. Apply and restart your GTK apps (make sure the appropriate GTK Styles and Themes settings are correct).

Colours KDE Control Centre module Hosted on Zooomr

Hope this helps someone!