Good day!
I have understood, how to customize fonts in the applications on main filesystem. That is, how to change font family to Liberation fonts for example and font size.
For GTK2 applications use Chtheme GTK/QT theme chooser.
For GTK3 applications (Firefox, Chromium) edit file /mnt/wkg/.session/etc/gtk-3.0/settings.ini
But, is it possible to customize fonts in the applications inside the containers, and in Flatpaks? Because Flatpaks have very small and blurry fonts. My eyes can not read them normally, they get tired very quickly. 
In the screenshot, there is an example. GIMP in the main filesystem has Liberation sans font, which I like. Gimp in the container has the default DejaVu sans font.
But KeePassXC, which has been installed using Flappi, has something small, blurry and almost unreadable.
Also, is it possible to configure font rendering (hinting, antialiasing) in EasyOS? There is such option in PuppyLinux, but I can not find it in EasyOS...