I install the oxygen application theme, to ensure qt5 apps also work with oxygen, I installed oxygen5 too. But after selecting the Oxygen theme, only qt6 apps applied the Oxygen theme. Qt5 apps are using the Fusion theme (I believe it’s the fallback theme). Did I miss anything? (KDE Plasma 6)
OK, I found that setting QT_STYLE_OVERRIDE=oxygen in /etc/environment works. But the change in Plasma color doesn’t apply to QT 5 apps.
A probably more accessible way is to install qt5ct and change themes there for QT 5 apps.