Set all Qt app's to use GTK+ font & theme settings
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 and add the following line:
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
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: 
21-Aug-2016: removed QT_STYLE_OVERRIDE mention, as qt5ct 0.25+ don't accept this anymore 29-July-2016: Updated method