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 wine-staging and winehq-staging are installed. I think wine-staging is a development branch of Wine but I don't know about winehq-staging.
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.

