This feature is not available right now. Please try again this video I demonstrate the blackberry 8530 running an OS6 theme. Theres a little lag on the theme here and there but if you want to try it I will leave the link here.
Sorry for the : 8530 OS6 Theme Theses are usually device an OS specific, meaning a theme for OS 6 will not work in OS 5.
Since you did not mention which theme you want, there isn' t much else I can say. I get from another forum, here the link for OTA I just download it from my devices and I save, so you can download this theme. Here' s screenshot Downl.