Created attachment 112293 [details] muon style Muon Discover not use engine style - style hardcoded. See example, on system used Kvantum engine. Kvantum use colors from kvantum theme and not applied systemsettings colors. But kvantum theme not worked fully for Discover, because some elements used some colors and elements from Breeze engine (possible) or hardcoded and use colorscheme from system settings. It's broken compability with non-Breeze engines. Other KDE apps not have this bug and use Engine theme and colors. You can see this on Dolphin on attached file and System Settings too.
Please report to Kvantum. Tested here, worked properly.