Hi @paulsnolan ,
In portal studio:
1. Click Themes > upload custom CSS > upload a file called "custom.css" or any name of your choice.
2. Paste the following code in the custom.css
.navbar-inverse .navbar-nav .open .dropdown-menu > li > a:hover, .navbar-inverse .navbar-nav .open .dropdown-menu > li > a:focus .navbar-inverse .navbar-nav .open .dropdown-menu > .active > a, .navbar-inverse .navbar-nav .open .dropdown-menu > .active > a:hover, .navbar-inverse .navbar-nav .open .dropdown-menu > .active > a:focus {
background-color: red !important;
color: greenyellow !important;
}
3. Press save > Sync Configuration > Browse website
When the portal is launched press ctrl + F5 on your screen to see the effect.
Output:

Hope it helps.
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