Set all Qt app's to use GTK+ font & theme settings
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13-March-2018: Updated contents & 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 qt5ct
2) Set environment variables in /etc/environment and add the following lines:
export QT_QPA_PLATFORMTHEME="qt5ct"
export QT_AUTO_SCREEN_SCALE_FACTOR=0
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.)
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]