/etc/java-8-openjdk/swing.properties is in openjdk-8-jre-headless 8u162-b12-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # uncomment to set the default look and feel to GTK
#swing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel
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