The QT 5.7.0 doesn’t support GTK theming, so after upgrade Qt related packages, the Qt based applications look… not really nice on GTK based desktops.

How to fix it:
sudo apt update
sudo apt install qt5-style-plugins

Then:
sudo echo "QT_QPA_PLATFORMTHEME=gtk2" >> /etc/environment
or
su
nano /etc/environment

and past into the file:
QT_QPA_PLATFORMTHEME=gtk2
then:
sudo reboot

That’s all. The first screen shows a Qt5 based application before, the second one after.

QMPlay2 before    QMPlay2 after