UIManager.setLookAndFeel(String полный_путь_к_классу);

try {
        UIManager.setLookAndFeel(new NimbusLookAndFeel());
    } catch (UnsupportedLookAndFeelException e) {
        e.printStackTrace();
    }

try {
        UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
    } catch (UnsupportedLookAndFeelException e) {
        e.printStackTrace();
    }

try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (ClassNotFoundException | InstantiationException |
                     IllegalAccessException
                    | UnsupportedLookAndFeelException e) {
                e.printStackTrace();
            }