Set all Qt app's to use GTK+ font & theme settings
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28-July-2016: Updated method
Introduction
For those of us that use Desktop Environments &/or Window Managers that use GTK+ , this fix is particularly helpful.
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
Results
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.
Links
This ArchWiki page goes into far more detail:
Uniform look for Qt and GTK applications [1]
Support
Following is a link to this page's forum counterpart where you can post any related feedback: [2]