The app is probably using QT5. I am using Wayland Gnome. On Manjaro, the app is using dark theme, but on EOS, it is light. There is no option to manually change the UI theme in the apps’ settings, so I think it is getting that from some sort of system settings, but I do not know what it is. I even used QT5CT to set a dark theme, but I cannot make Ocenaudio dark on EOS.