Set all Qt app's to use GTK+ font & theme settings
28-July-2016: Updated method
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 (adding the lines)
3) Restart the system
4) Start qt5ct from terminal and select gtk2
5) Start one more time qt5ct from terminal with sudo rights (for qt apps which required sudo rights like manjaro-settings-manager) 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: