Set all Qt app's to use GTK+ font & theme settings
13-March-2018: Updated contents & 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 qt5ct
2) Set environment variables in /etc/environment and add the following lines:
3) Restart the system
4) Start qt5ct
5) In the first tab Appearance choose Standard Dialogs: default (some people get crashes if they choose GTK3)
Because we used /etc/environment to set the environment variables back in step 2), after you reboot, both the user & root accounts will be able to benefit from these changes.
(If you find that certain widget styles are missing, you can see if qt5-styleplugins brings a fix for you.)
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: