Hover Background Color Disappears on Post-Back

On one of my Web pages I have included a treeview control in an Update Panel. It works fine except for when the page is updated from a post-back generated by a click on a treeview node the treeview control looses its yellow background hover color. If I remove the update panel the background hover color works through post-backs. I would appreciate any input on how to make the background hover color survive the postback with the treeview inside an update panel.