Bug 251843: Add alternating background to about:config. r=mconnor

toolkit/components/viewconfig/content/config.xul

@@ -126,8 +126,8 @@
             class="sortDirectionIndicator"
             persist="hidden width ordinal sortDirection"/>
       </treecols>
-  
-      <treechildren id="configTreeBody" ondblclick="if (event.button == 0) ModifySelected();"/>
+
+      <treechildren id="configTreeBody" alternatingbackground="true" ondblclick="if (event.button == 0) ModifySelected();"/>
     </tree>
   </vbox>
 </deck>