As indicated in this answer, Wine-staging now has an option for GTK theming, and it works as indicated there.
Adding a Wine-staging version to a virtual drive in Playonlinux , the "Staging" tab appears in Wine configuration for that drive, but "Enable GTK Theming" is grayed-out.
How could I get the GTK theming to work in Playonlinux?
Following the linked solution, both
winehq-staging are installed. I think
wine-staging is a development branch of Wine but I don't know about
winehq-staging needs and installs
wine-staging in order to work, but I don't think Playonlinux installs
winehq-staging and that may be the problem.
Having that in Playonlinux would be an advantage because of the easier of using different Wine versions for different programs, as gtk theming may not really represent a progress for all windows programs.