Default css rule for bootstrap 3 for background hover dropdown
.dropdown-menu>li>a:hover { color: #262626; text-decoration: none; background-color: #f5f5f5; } //somewhat on line 6 or 7
To change this, you can add your custom css rule, and I recommend that you add your own id or targeting class to the drop-down list, rather than overriding the default value using your own selector so that other dropdows don't get hurt.
.your-class>li>a:hover { background-color: red !important; }
Hope this helps.
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