GUIChooser starts but not Experimenter or Explorer

The GUIChooser starts, but Explorer and Experimenter don't start and output an Exception like this in the terminal:
 /usr/share/themes/Mist/gtk-2.0/gtkrc:48: Engine "mist" is unsupported, ignoring
 ---Registering Weka Editors---
        at weka.gui.explorer.PreprocessPanel.addPropertyChangeListener(
        at javax.swing.plaf.synth.SynthPanelUI.installListeners(
        at javax.swing.plaf.synth.SynthPanelUI.installUI(
        at javax.swing.JComponent.setUI(
        at javax.swing.JPanel.setUI( 
This behavior happens only under Java 5/6 and Gnome/Linux, KDE doesn't produce this error. The reason for this is, that Weka tries to look more "native" and therefore sets a platform-specific Swing theme. Unfortunately, this doesn't seem to be working correctly in Java 5/6 together with Gnome. A workaround for this is to set the cross-platform Metal theme.

In order to use another theme one only has to create the following properties file in ones home directory:
With this content:
More information can be found in this Weka list post or here.