Take a look at
https://wiki.archlinux.org/index.php/qt one way is using
the plugin [1], that not necessarily does a good job, but, hey, that is
nowadays Linux, it's absolutely inconsistent.
[1]
No Rosegarden here, but Rui's apps are based upon Qt, too:
[rocketmouse@archlinux ~]$ qjackctl
qt5ct: using qt5ct plugin