This would require a change to Splunk's core css that would not be persisted if an update occurs.
However if you want it changed for a particular dashboard that you happen to have the ability to modify the css of, then the following code may help you:
.navbar .nav>li.active.dropdown>a.dropdown-toggle:hover, .navbar .nav>li>a:hover
color: #ffffff; /change your color here/
background: #444; /change your background color here/