java.lang.NoClassDefFoundError: sun / swing / plaf / synth / SynthUI

Estou usando o Seaglass Look and Feel para meu código Java Swings. Mas continuo recebendo esse erro.

    Exception in thread "main" java.lang.NoClassDefFoundError: sun/swing/plaf/synth/SynthUI
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:278)
    at javax.swing.SwingUtilities.loadSystemClass(SwingUtilities.java:1873)
    at javax.swing.UIDefaults.getUIClass(UIDefaults.java:686)
    at javax.swing.UIDefaults.getUI(UIDefaults.java:758)
    at javax.swing.UIManager.getUI(UIManager.java:1013)
    at javax.swing.JRootPane.updateUI(JRootPane.java:483)
    at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1230)
    at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1245)
    at javax.swing.SwingUtilities.updateComponentTreeUI(SwingUtilities.java:1221)
    at jsupport.lookandfeel.LookAndFeel.changeLookandFeel(LookAndFeel.java:246)
    at jsupport.lookandfeel.LookAndFeel.<init>(LookAndFeel.java:22)
    at jsupport.lookandfeel.Main.main(Main.java:11)
Caused by: java.lang.ClassNotFoundException: sun.swing.plaf.synth.SynthUI
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    ... 25 more

Pesquisei um pouco e descobri que o Seaglass não funciona com o Java 7 (minha versão atual) e, portanto, o problema. Mas estou procurando uma maneira de fazê-lo funcionar ou se alguém tiver alguma demonstração que funcione perfeitamente com a versão Java 7, seria muito útil.

Meu código:

   private void changeLookandFeel() {
    try {

        UIManager.removeAuxiliaryLookAndFeel(UIManager.getLookAndFeel());
        SyntheticaLookAndFeel.setWindowsDecorated(false);
        UIManager.setLookAndFeel(UIMANAGER_STRING);
         UIManager.setLookAndFeel("de.javasoft.plaf.synthetica.SyntheticaBlueIceLookAndFeel");

         for (int i = 0; i < LookAndFeel.getFrames().length; ++i) {
            SwingUtilities.updateComponentTreeUI(LookAndFeel.getFrames()[i]);
          SwingUtilities.updateComponentTreeUI(this);
       }
        SwingUtilities.updateComponentTreeUI(this);

    } catch (Exception ex) {
        try {
           UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
           UIManager.setLookAndFeel("de.javasoft.plaf.synthetica.SyntheticaBlackEyeLookAndFeel");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

Os frascos que estou usando