Looks like you put out a new version which has a publisher specified.

Anyway, I did have the theme editor crash when following the ListBox part of the tutorial.
It happened when I was trying to edit the preferred height of the ListBox (yeah that's not a step in the tutorial, I wanted to see how it behaved).
(click image in link for full size)
http://postimage.org/image/bolm55agh/
Also, one other minor comment:
When adding a new node (theme etc), I think the theme editor used to focus the auto-generated Name text box automatically. It's not doing that now. Overall the editor is pretty darn cool, though!
Edit: Actually, editing preferred width is also causing a crash after I load this theme (and select ListBox widget obviously). This is before even adding the vscrollbar theme that has the maxHeight parameter (which I think was a mistake since I really needed to add that to the listbox). So I guess that isn't the cause... but now I'm not sure what is.
Code:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE themes PUBLIC "-//www.matthiasmann.de//TWL-Theme//EN" "http://hg.l33tlabs.org/twl/raw-file/tip/src/de/matthiasmann/twl/theme/theme.dtd">
<themes>
<images file="widgets.png">
<area name="box" xywh="1,1,42,34" nocenter="true" splity="T5,B5" splitx="L13,R13"/>
<area name="box bg" xywh="44,3,38,30" splity="T9,B9" splitx="L9,R9" border="2"/>
<area name="hscrollbar.thumb.overlay" xywh="158,24,5,10" center="true"/>
<area name="vscrollbar.thumb.overlay" xywh="161,15,10,5" center="true"/>
<composed name="panel-blue">
<alias ref="box bg" tint="#6AACDE"/>
<alias ref="box" tint="#45CD64"/>
</composed>
<select name="button.background" sizeOverwriteV="26">
<alias ref="box" if="pressed|selected" tint="#DCFD37" inset="2,2,-2,-2"/>
<alias ref="box" if="hover" tint="#80A2E9"/>
<alias ref="box" tint="#89A9EC"/>
</select>
<select name="hscrollbar.button" sizeOverwriteH="22" sizeOverwriteV="22">
<alias ref="panel-blue" if="pressed"/>
<alias ref="panel-blue" if="hover"/>
<alias ref="panel-blue" tint="#A9C1F1"/>
</select>
<select name="vscrollbar.button" sizeOverwriteH="22" sizeOverwriteV="22">
<alias ref="panel-blue" if="pressed"/>
<alias ref="panel-blue" if="hover"/>
<alias ref="panel-blue" tint="#A9C1F1"/>
</select>
<composed name="panel-grey">
<alias ref="box bg" inset="2" tint="#BFC0C2"/><alias ref="box" tint="#6AABDE"/>
</composed>
<composed name="panel-lightblue">
<alias ref="box bg" inset="2" tint="#3CC9EB"/>
<alias ref="box" tint="#6AAADE"/>
</composed>
</images>
<fontDef name="normal" filename="font.fnt" color="white" default="true">
<fontParam if="error" color="red"/>
<fontParam if="warning" color="orange"/>
<fontParam if="disabled" color="gray"/>
<fontParam if="textSelection" color="blue"/>
</fontDef>
<fontDef name="button.font" filename="font.fnt" color="#89A9EC">
<fontParam offsetY="2" offsetX="2" color="#DDFD37" if="pressed|selected"/>
<fontParam color="#BED0F4" if="hover"/>
</fontDef>
<inputMapDef name="-defaultInputMap">
<action name="selectAll">ctrl A</action>
<action name="selectAll">cmd A</action>
<action name="cut">ctrl X</action>
<action name="cut">cmd X</action>
<action name="copy">ctrl C</action>
<action name="copy">cmd C</action>
<action name="paste">ctrl V</action>
<action name="paste">cmd V</action>
</inputMapDef>
<theme name="-defaults">
<param name="background"><image>none</image></param>
<param name="overlay"><image>none</image></param>
<param name="font"><font>normal</font></param>
<param name="textAlignment"><enum type="alignment">left</enum></param>
<param name="minWidth"><int>0</int></param>
<param name="minHeight"><int>0</int></param>
<param name="maxWidth"><int>0</int></param>
<param name="maxHeight"><int>0</int></param>
<param name="inputMap"><inputMap>-defaultInputMap</inputMap></param>
</theme>
<theme name="label" ref="-defaults" allowWildcard="true">
</theme>
<images file="graphics/simple.png" format="RGBA">
<area name="-borderG" border="3" xywh="60,0,9,9" splitx="L3,R3" splity="T3,B3"/>
<area name="white" xywh="23,3,1,1"/>
</images>
<theme name="tooltipwindow" ref="-defaults">
<param name="background"><image>-borderG</image></param>
<param name="border"><border>background.border</border></param>
<param name="fadeInTime"><int>0</int></param>
<theme name="label" ref="label">
<param name="font"><fontDef filename="font.fnt" color="black"/></param>
</theme>
</theme>
<theme name="button" ref="-defaults">
<param name="font"><font>button.font</font></param>
<param name="background"><image>button.background</image></param>
<param name="border"><border>10,0</border></param>
</theme>
<theme name="hscrollbar" ref="-defaults">
<theme name="leftbutton" ref="-defaults">
<param name="background"><image>hscrollbar.button</image></param>
</theme>
<theme name="rightbutton" ref="-defaults">
<param name="background"><image>hscrollbar.button</image></param>
</theme>
<theme name="thumb" ref="-defaults">
<param name="background"><image>hscrollbar.button</image></param>
<param name="overlay"><image>hscrollbar.thumb.overlay</image></param>
<param name="minWidth"><int>40</int></param>
</theme>
<param name="background"><image>panel-lightblue</image></param>
<param name="scaleThumb"><bool>false</bool></param>
</theme>
<theme name="vscrollbar" ref="-defaults">
<theme name="upbutton" ref="-defaults">
<param name="background"><image>vscrollbar.button</image></param>
</theme>
<theme name="downbutton" ref="-defaults">
<param name="background"><image>vscrollbar.button</image></param>
</theme>
<theme name="thumb" ref="-defaults">
<param name="background"><image>vscrollbar.button</image></param>
<param name="overlay"><image>vscrollbar.thumb.overlay</image></param>
<param name="minHeight"><int>40</int></param>
</theme>
<param name="background"><image>panel-lightblue</image></param>
<param name="scaleThumb"><bool>false</bool></param>
</theme>
<theme name="togglebutton" ref="button"/>
<theme name="widget" ref="-defaults">
<param name="background"><image>panel-grey</image></param>
</theme>
<theme name="listbox" ref="label">
<param name="background"><image>panel-blue</image></param>
<theme name="display" ref="label"/>
<param name="border"><border>10,5</border></param>
</theme>
</themes>