I always liked their original UI - Photon[1][2]. Very lightweight and fast. Also a distinct and consistent style. I understand why they dropped it in favor of Qt and later Web technologies, but it's still a big loss.
[1] https://www.qnx.com/developers/docs/6.5.0SP1.update/com.qnx....
[2] https://www.mikecramer.com/qnx/momentics_nc_docs/photon/prog...
Yes while it makes sense to reuse stuff that is already being built, my heart sank when I saw the screenshot while expecting seeing the photon microgui which to me was the nicest skeuomorphic one.
I was expecting to see that, I ended up looking up some old LiteStep themes [1][2] for my fix
[1] https://www.wincustomize.com/explore/litestep/6/
[2] https://www.wincustomize.com/explore/litestep/292/