.nav {
    display: flex;
    align-items: stretch;
    position: fixed; 
    bottom: 0; 
    width: 100%; 
    height: 50px; 
    background: #42bd56; 
    color: #fff;
    span {
        flex: 1;
        display: flex;
        align-items: center;
        justify-content: center;
    }
}