Update generic wxTreeCtrl appearance when theme changes

See #18823

include/wx/generic/treectlg.h

@@ -351,6 +351,10 @@ protected:
     virtual wxSize DoGetBestSize() const wxOVERRIDE;
 
 private:
+    bool m_hasExplicitFont;
+
+    void OnSysColourChanged(wxSysColourChangedEvent&);
+
     // Reset the state of the last find (i.e. keyboard incremental search)
     // operation.
     void ResetFindState();