Hi Jesus,
To implement such thing, please open the custom.css file in folder: templates/your_template/css. If you don't see the file, create such file and add the following css rule:
.navbar-default .navbar-nav > .open > a, .navbar-default .navbar-nav > .open > a:hover, .navbar-default .navbar-nav > .open > a:focus {
background-color: #f8f8f8;
}
The background-color is the color when you hovering a menu item. You can change the color code to meet your expectation.
Hope it helps you.
Gardner.