Disable GTK2-specific code to change theme when building wxGTK3.

It doesn't work with GTK3, it causes deprecation warnings, and allowing
an app to change its theme is a dumb idea


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72925 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/app.cpp

@@ -199,6 +199,10 @@ wxApp::~wxApp()
 
 bool wxApp::SetNativeTheme(const wxString& theme)
 {
+#ifdef __WXGTK3__
+    wxUnusedVar(theme);
+    return false;
+#else
     wxString path;
     path = gtk_rc_get_theme_dir();
     path += "/";
@@ -219,6 +223,7 @@ bool wxApp::SetNativeTheme(const wxString& theme)
     gtk_rc_reparse_all_for_settings(gtk_settings_get_default(), TRUE);
 
     return true;
+#endif
 }
 
 bool wxApp::OnInitGui()