/etc/java-7-openjdk/swing.properties is in openjdk-7-jre-headless 7u181-2.6.14-1~deb8u1.
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
|