Generally, you can change the look and feel either by passing command line arguments, e.g. netbeans --laf javax.swing.plaf.metal.MetalLookAndFeel or do so programmatically on startup in a module’s ModuleInstall class. Generally if you’re installing a custom look and feel, you probably want to have a separate module to do that. Some look and feels provide NetBeans modules to make it easy to use them.

If you want to test running with a different look and feel during development of your application, and you know it will be on the application’s classpath, see the example in HowToPassCommandLineArgumentsToANetBeansPlatformApplicationWhenRunInsideTheIDE for how to include --laf in the runtime arguments to your module suite.