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).
Hope this helps someone!