SUMMARY If you set your application theme to Oxygen or Plastic in preferences, flatpak programs will still use the default Breeze theme. This is because flatpak apps can't see the installed qt themes in the system, and only can use themes installed as flatpak. At the moment the only themes that flatpak programs can use are Breeze, Windows, and Fusion. STEPS TO REPRODUCE 1. Set oxygen or plastic theme in preferences 2. Open a qt flatpak app OBSERVED RESULT Default breeze theme is used EXPECTED RESULT Flatpak apps using the theme configured in preferences, same as the system installed programs, so they look integrated with the rest of the system. ADDITIONAL INFORMATION Another solution may be to create separate flatpak packages for Oxygen and Plastic. Or allow the KDE runtime to use system installed themes.
Just to correct my bad writing in the "Additional Information part". I´d just like to clarify that the expected solution would be to include the plastic and oxygen themes in the KDE flatpak runtime. But another possible solutions would be to create separate flatpak packages for Oxygen and Plastic. Or allow the KDE runtime to use system installed themes.