If you install a Theme that was not made for your specific OS/Device it could brick your device.
Themes really aren't malicious, although I am not sure if they could be.
The 9630 has NO third party themes out there, so I would not install any.
8830 -> 8330 -> 9550 -> 9650
Just think about how far BlackBerries have come from then till now... And what else is coming.
Follow me on Twitter