Set all Qt app's to use GTK+ font & theme settings
QGtkStyle has been removed from qt5-base 5.7.0 & added to qt5-styleplugins which is an AUR package. So since Qt 5.7 you need to install qt5-styleplugins if you are using a GTK+ based DE/WM. If you use both GTK2 & GTK3 applications you may need to find a compatible theme.
For those such as myself that run 1080p or higher screen resolutions on largish monitors it can be quite a strain on the eyes when you can't control the size of the fonts that an application uses.
This fix makes your Qt app's use your GTK+ theme (colours & such) & your chosen font settings.
How do you do it?
1) Install qt5-styleplugins and qt5ct 2) Set environment variable in /etc/environment: QT_QPA_PLATFORMTHEME=qt5ct 3) Restart the system 4) Start qt5ct with sudo rights (very important) and select gtk2
After saving the file & rebooting, I then, with trepidation, opened the Qt 5.7 program called PkgBrowser... It looked just like my other GTK+ app's on my Openbox system (there is no accounting for taste!). I was very happy.
Here is a picture of the Qt 5.5 PkgBrowser program open full screen on my 1080p Openbox (GTK2) box. It is using my custom dark theme & my large font settings. My eyes love it.
This ArchWiki page goes into far more detail:
Uniform look for Qt and GTK applications 
Following is a link to this page's forum counterpart where you can post any related feedback: