ok, ya he solucionado el problema.
En vez de invocar el id del div, invoco el parametro del Struts, asi:
Código PHP:
<div class="wrapGreen" onmouseout="javascript:showlayer('<s:property value='#counter.count'/>');" onmouseover="javascript:showlayer('<s:property value='#counter.count'/>');">
<div class="card">
<s:url action="viewPublicTheme" id="url" escapeAmp="false">
<s:param name="crudTheme.id" value="id"/>
<s:param name="crudThemeEnhancer.id" value="themeEnhancer.id"/>
</s:url>
<a href="<s:property value="#url"/>">
<img class="listPublicThemesImages" alt="Picture" src="ShowImage.action?id=<s:property value="id"/>&imageType=themeThumb"/>
</a><br>
</div>
<div id="<s:property value='#counter.count'/>" class="greenMask">
<div id="themeTitle" class="brodThemeTitle">
<s:property value="themeName"/>
</div>
<img src="/Fixafesten/images/frame_big_inner.gif" border="0"/>
</div>
</div>
Facil!!!