Generally, there’s no problem theming Paraview by editing QT settings. The problem/issue is to get Paraview to read the obtained qtconfig file. If you used e.g. the version packaged with Ubuntu/Debian linux, and qt5ctrl to configure the entire QT, then Paraview will respect it and use the dark theme.
However, if an OS uses the binary boundle downloaded from paraview.org, it’s a different pair of shoes, and enforcing a QT config file is more involved.