Scegliere il LookAndFeel:

String lookAndFeel = null;
lookAndFeel = UIManager.getCrossPlatformLookAndFeelClassName();
lookAndFeel = UIManager.getSystemLookAndFeelClassName();
lookAndFeel = "com.sun.java.swing.plaf.motif.MotifLookAndFeel";

LookAndFeel esistenti di default in java:

Base: UIManager.getCrossPlatformLookAndFeelClassName();
System: UIManager.getSystemLookAndFeelClassName();
Motif: com.sun.java.swing.plaf.motif.MotifLookAndFeel";
GTK: com.sun.java.swing.plaf.gtk.GTKLookAndFeel";

Settare il LookAndFeel:

UIManager.setLookAndFeel(lookAndFeel);

Per il LookAndFeel Metale si posso scegliere diversi temi:

DefaultMetal: MetalLookAndFeel.setCurrentTheme(new DefaultMetalTheme());
Ocean: MetalLookAndFeel.setCurrentTheme(new OceanTheme());

Per modificare il LookAndFeel anche della finestra (di default usa quella dell'SO):

JFrame.setDefaultLookAndFeelDecorated(true);