You can change the client program's Look & Feel (Skin) by change the Look & Feel option at (Tools | GlobalOptions | Display). It will take effect after program's restart.

If you want to install third party Java Look &Feel for your system, you can have a number of choices, such as the JGoodies (com.jgoodies.looks.plastic.PlasticLookAndFeel, com.jgoodies.looks.plastic.Plastic3DLookAndFeel, com.jgoodies.looks.plastic.PlasticXPLookAndFeel, com.jgoodies.looks.windows.WindowsLookAndFeel), Mac OS X (ch.randelshofer.quaqua.tiger.Quaqua14TigerCrossPlatformLookAndFeel), etc.  Please follow the following steps to install third party's Java Look & Feel: (Warning: For EXPERT only! backup swing.property file before you try)
i. Copy the L&F jar to JAVA_INSTALL_DIR/jre/lib/ext/,
ii. use this tool to help you add a Look & Feel class in Java, or you can modify the Java Swing config file directly as follows:
- Edit the file JAVA_INSTALL_DIR/jre/lib/swing.properties
- Add or set the line "swing.installedlafs=motif,metal,...,L&FName"
- Add the line "swing.installedlaf.L&FName.name=TheName"
- Add the line " swing.installedlaf.L&FName.class = L&F ClassName"