Make sure you have a dark theme installed
and activated before configuring qt apps!
nano ~/.config/Trolltech.conf
add lines:
[Qt]
style=GTK+
For Debian/Linux Mint/Ubuntu:
sudo apt-get install qt5-style-plugins
For Arch Linux:
sudo pacman -S qt5-styleplugins
sudo nano /etc/environment
add lines:
QT_QPA_PLATFORMTHEME=gtk2
also, this line may be added in case you still have light-colored apps after reboot:
QT_STYLE_OVERRIDE=gtk2