The synth package level documentation contains a typo in the text where it shows how to load an xml theme. It currently reads:

SynthLookAndFeel laf = new SynthLookAndFeel();
laf.load(MyClass.class.getResourceAsStream("laf.xml",
         MyClass.class));
UIManager.setLookAndFeel(laf);

It should read:

SynthLookAndFeel laf = new SynthLookAndFeel();
laf.load(MyClass.class.getResourceAsStream("laf.xml"),
         MyClass.class);
UIManager.setLookAndFeel(laf);