Hi
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For this add the below css code inside the Appearance >> customizer >> Find the additional css box and add the below css code inside the box.
.navbar .navbar-nav > .open > a, .navbar .navbar-nav > .open > a:hover, .navbar .navbar-nav > .open > a:focus, .navbar .navbar-nav > li > a:hover,
.navbar .navbar-nav > li > a:focus, .navbar-default .navbar-nav > .active > a, .navbar-default .navbar-nav > .active > a:hover, .navbar-default .navbar-nav > .active > a:focus,
.navbar-default .navbar-nav > .current_page_item > a,
.navbar-default .navbar-nav > .current_page_item > a:hover, .navbar-default .navbar-nav > .current_page_item > a:focus
{
background-color: #70f228;
}
Change the color code value as your need.
Let us know for any confusion.
Thanks