Consider the screenshot:

The image comes from:

JabRef 5.0-dev--snapshot--2019-05-25--master--3ebc59a76
Linux 5.1.4-1-default amd64 
Java 1.8.0_211 (Oracle java, build 1.8.0_211-b12)

Since the same issue is occurring in any multiple 4.x JabRef versions, the issue seems to be related to the UI technology JavaFX on Linux (sorry, if my question is outside the context of JabRef, feel free to ban it).

On the other hand, fonts in JabRef 3.8.2 looks OK.

From the web, there are some possible solutions to this issue, but all in Java code and not external ones such as fixing the bash-variable _JAVA_OPTIONS properly. Of course, the bash-command

java -Dawt.useSystemAAFontSettings=on -Dswing.aatext=true -jar JabRef--master--latest.jar

or playing with JabRef Menu: Options » Preferences » Appearance, doesn't work.

The KDE Fonts system parameters as Anti-aliasing, Sub-pixel rendering and Fonts DPI are setting as Vendor Default and fonts on any other non-JavaFX application looks OK.

Could you give me some suggestions in order to improve font rendering in JabRef/JavaFX, please?