Estoy usando Synthetica Look and Feel en mi aplicación y funciona bien en Eclipse. Pero cuando exporto el archivo Jar y lo ejecuto obtuve estos errores:

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: sun/swing/plaf/synth/SynthUI
at de.javasoft.plaf.synthetica.SyntheticaDefaultLookup.getDefault(SyntheticaDefaultLookup.java:105)
at sun.swing.DefaultLookup.get(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.getInputMap(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.installKeyboardActions(Unknown Source)
at javax.swing.plaf.basic.BasicButtonUI.installKeyboardActions(Unknown Source)
at javax.swing.plaf.basic.BasicButtonUI.installUI(Unknown Source)
at javax.swing.JComponent.setUI(Unknown Source)
at javax.swing.AbstractButton.setUI(Unknown Source)
at javax.swing.JButton.updateUI(Unknown Source)
at javax.swing.AbstractButton.init(Unknown Source)
at javax.swing.JButton.<init>(Unknown Source)
at javax.swing.JButton.<init>(Unknown Source)
at de.javasoft.plaf.synthetica.SyntheticaLookAndFeel.installCompatibilityDefaults(SyntheticaLookAndFeel.java:925)
at de.javasoft.plaf.synthetica.SyntheticaLookAndFeel$4.propertyChange(SyntheticaLookAndFeel.java:582)
at java.beans.PropertyChangeSupport.fire(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at javax.swing.event.SwingPropertyChangeSupport.firePropertyChange(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at javax.swing.UIManager.setLookAndFeel(Unknown Source)
at com.evaluator.MainWindow.<init>(MainWindow.java:85)
at com.evaluator.Main$1.run(Main.java:15)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
   Caused by: java.lang.ClassNotFoundException: sun.swing.plaf.synth.SynthUI
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 35 more

No sé dónde está el problema exactamente. Esta es la forma en que estoy aplicando la apariencia y apariencia de Synthetica Blue Steel:

try 
    {
      UIManager.setLookAndFeel(new SyntheticaBlueSteelLookAndFeel());
      UIManager.put("Synthetica.rootPane.titlePane.title.center", true);
      UIManager.put("Synthetica.translucency4DisabledIcons.enabled", true); 

    } 
    catch (Exception e) {}