webdev/css/flex.css

.bigbox{
    width: 100%;
    border: 2px solid yellowgreen;
    padding: 1em 1em 0 1em;
    display: flex;
    justify-content: center;
    align-items: center;
}
.box{
    width: 150px;
    height: 150px;
    margin: 0 1em 1em ;
    background-color: yellowgreen;
}
.grow{
  flex-grow: 1;
}