To style the current page you would need a rule
#nav-header-mini .current-menu-item a {text-decoration:underline;}
So you can add it to the current CSS+ box with
{float:left;} %selector% a {padding:0 7px;font-size: 125%;} %selector% .current-menu-item a {text-decoration: underline;}
This rule in the CSS+ box would be the same as the rules below in Main Options > FOnts & Custom > Custom CSS Rule box
#nav-header-mini {float:left;}
#nav-header-mini a {padding:0 7px;font-size: 125%;}
#nav-header-mini .current-menu-item a {text-decoration: underline;}