The default Look&Feel (skin) can be changed by editing the marvin.properties file (in the user's home/chemaxon directory) and inserting/changing the "skin=..." line to the following:
Metal Look&Feel
skin=javax.swing.plaf.metal.MetalLookAndFeel
Nimbus Look&Feel
skin=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel
CDE/Motif Look&Feel
skin=com.sun.java.swing.plaf.motif.MotifLookAndFeel
Windows Look&Feel
(Default on Windows, can be deleted) skin=com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Windows Classic Look&Feel
skin=com.sun.java.swing.plaf.windows.WindowsClassicLookAndFeel
Jgoodies Plastic XP Look&Feel
skin=com.jgoodies.looks.plastic.PlasticXPLookAndFeel
Jgoodies Plastic Look&Feel
skin=com.jgoodies.looks.plastic.PlasticLookAndFeel