css flex center

.primary-nav {
    display:-webkit-flex;
    display:flex;
    list-style-type:none;
    padding:0;
    justify-content:flex-end;
}

.left {
    margin-right:auto;
}

align-items: flex-start | flex-end | center | baseline | stretch

.container {
  display: flex;
  align-items: flex-start;
}

.box.flex {
  display: flex;
  justify-content: center;
}