How to add custom classes to more than one separate menu’s a link in wordpress?

Just use CSS to style your #nav li block.

This is quick, but would make your links into buttons.

    #nav li {
        list-style: none;
        background: #F00;
        width: 120px;
        margin-right: 60px;
        padding: 10px;
        text-align: center;
}

you can add a hover class too like #nav li:hover