Sorry, i really cannot see a technical reason for one white window amidst a otherwise dark GUI. Or the other issues like different scrollbar designs. I know that there is still a lot of SX-Code left, especially in the dialogs, but I have no problem with that, those are actually fine.
It is mostly the stuff since C7 when they redesigned a lot that is inconsistent, so it cannot be because of an old codebase. Yes, Cubase was never the prettiest or most elegant DAW on the market, but things have gotten worse in the past years.