Nextcloud Client not accepting OS Theme in Linux Mint 19


i switched from Linux Mint 18.3 to 19, and noticed an odd behaviour:
I like the dark theme a lot since it is easier on the eyes. On LM 18.3 the client used whatever theme was selected by the OS (see screenshot)

Under 19, the client does not use any theme, not even the standard theme from LM (see screenshot)

I thought about posting this on but it is readonly now, so i thought i should post here.
Can someone explain why this happens? It is a clean install in both cases …



I also discovered this problem in a few other programs, so i decided to post this on the Linux Mint forums instead:

It seems to be a qt-problem, and can be fixed (at least for nextcloud-client) by the following:

    sudo apt install qt5ct qt4-qtconfig
    qt5ct -> Style: GTK2 -> Apply, OK
    qtconfig-qt4 -> GUI Style -> GTK+ -> File -> Save, Exit
    sudo reboot