This is a behavior of Windows Installer. Generally I've seen it use an unusually small font instead of one that's entirely boxes, but in short, Microsoft doesn't support this. If you want to make it work for certain, you'd need to use an external UI (preferably embedded, because that avoids a lot of the work required to get the Uninstall key updates to work correctly - see Heath Stewart's blog for more details on that if you create an external UI and can't go embedded).
