Remove unsupported "align" attr and use "pack" instead.

b=112991; r=sgehani; sr=hewitt
(checking in for Colin Phillips <colinp@oeone.com>)

xpfe/components/prefwindow/resources/content/pref-themes.xul

@@ -74,7 +74,7 @@
 
   <separator class="thin"/>
   
-  <hbox id="previewImageContainer" class="inset" align="center" >
+  <hbox id="previewImageContainer" class="inset" pack="center">
     <image id="previewImage"/>
   </hbox>