Wine has an option for GTK theming if you use the wine-staging version.
I've installed GTK3 on macOS through:
brew install gtk+3
However, the option remains disabled. Is there a way this can be done on macOS?
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As I’d said on Winehq, the only way to enable gtk themes required that wine-staging is built from source with this library available.
As you didn’t provide the version of macOS your running there’s little more information that can be provided, also from what was posted on Winehq you seem reluctant to compile wine from source making this impossible to achieve.