As for the rest, theme design is not UX design at all.
I don't remember the old audacity ever matching my system theme so eh, what i do remember is it being a pain of browsing through tutorials and documentation to be able to do what i want to do with it. If i can make it do what i want it to do, without thinking too much about it, and no functionality is lacking, i'm not going to complain.
Sure, i'll have to relearn some things, but people who will start with it today won't have to spend as much time reading and rereading scarce documentation, i think it's a good thing?

os-wise, i don't know people seriousely daily driving things that qt does not support. Or if they do (i.e. my grandma), they have long since switched from xp to some linux distro with the help of some it person (which wasn't even me, how sad).
But i'm pretty sure you can find a qt qpa that will allow audacity to run on xp or even 95. But that's a stretch, if you really do use such outdated oses, you likely also use just as outdated software. And then again, 3.7 (which isn't out yet) will still be supported for a while