gnome - Qt apps stopped inheriting GTK themes - Ask Ubuntu

Install qt5-style-plugins from the Ubuntu repositories with sudo apt install qt5-style-plugins.

Then add export QT_QPA_PLATFORMTHEME=gtk2 to ~/.profile by echo "export QT_QPA_PLATFORMTHEME=gtk2" >> ~/.profile. This will theme all apps started as user.

If you prefer to apply the theming system-wide (for user and root apps), then use sudo sh -c "echo 'export QT_QPA_PLATFORMTHEME=gtk2' >> /etc/environment".