I can’t see how that happened. Certainly not intentional. Presumably an inadvertent knock-on effect from some other change.
I’ll fix it for next release, but for now you can add this to your custom style sheet (mystyles.css)
.ui-menu .ui-menu-item a.ui-state-hover,.ui-menu .ui-menu-item a.ui-state-active {
background: none;
}
NOTE for anyone else – this is a colors theme issue only.