The screenshots in the handbook were taken with the Nimbus Look&Feel. You can switch to Nimbus by starting ScenePainter like this:

java -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel -jar scenepainter.jar

Be warned, though. As slick as Nimbus may look, it is a performance hog and should only be used on a fast computer.