It also says “free of installer bundles” while (at least in past, don’t know about now) their default windows installer was including ads (source)
P.S.: If you want to disable the in-app ads, here’s how to: https://gist.github.com/yunooooo/87564a0293a1c7a53fb3e233a604c638
I was mistaken and said KDE rather than GTK theme but you just need to add this add an environment variable before the operator on the desktop shortcut file and it should work
_JAVA_OPTIONS="-Dawt.useSystemAAFontSettings=on -Dswing.aatext=true -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel -Dswing.crossplatformlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel ${_JAVA_OPTIONS}"