Можно воспользоваться опцией --laf:

./netbeans --laf com.sun.java.swing.plaf.gtk.GTKLookAndFeel

Известные интерфейсы, идущие по-умолчанию в комплекте с NetBeans:

• Metal: javax.swing.plaf.metal.MetalLookAndFeel
• Nimbus: com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel
• CDE/Motif: com.sun.java.swing.plaf.motif.MotifLookAndFeel
• Mac OS X: com.apple.laf.AquaLookAndFeel
• Windows: com.sun.java.swing.plaf.windows.WindowsLookAndFeel

См. также:

Улучшаем интерфейс Java-приложения

http://habrahabr.ru/blogs/java/117430/

Еще вариант:

You aren't limited to the L&Fs supplied with the Java platform. You can use any L&F that is in your program's class path. External L&Fs are usually provided in one or more JAR files that you add to your program's class path at runtime. For example:

java -classpath .;C:\java\lafdir\customlaf.jar YourSwingApplication

Еще вариант:

First some background… Java look and feels can be specified on the command line using the property “swing.defaultlaf”. This property gives the fully qualified classname of the look and feel to load.

java -Dswing.defaultlaf=com.example.foo.FooLookAndFeel -jar app.jar

Как подключить TinyLaf:

Martin, you can create your own theme, save it as Default.theme and then you have to put it in the classpath root.

To do this, you can put the Default.theme file in the root of tinylaf.jar and then store the jar in the lafs folder. This has worked for me.