[Bug 48155] New: winecfg: Enable GTK Theming option is always grayed out

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=48155

Bug ID: 48155
           Summary: winecfg: Enable GTK Theming option is always grayed
                    out
           Product: Wine-staging
           Version: 4.20
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: -unknown
          Assignee: [e-mail]
          Reporter: [e-mail]
                CC: [e-mail], [e-mail]
      Distribution: ---

Created attachment 65772
  --> https://bugs.winehq.org/attachment.cgi?id=65772
Fix for the issue

When going to the Staging tab in winecfg, the GTK theming option is grayed out,
even though wine was compiled with gtk3 enabled.
Enabling GTK theming in the registry still works.

Attached is a patch that fixes the issue.