body {
    margin: 0;
}


.parent {
    display: flex;
    justify-content: center;
    box-sizing: border-box;
    background-color: midnightblue;
    padding: 10px;
    width: 60%;
    margin-inline: auto;
    height: 100vh;
}

.child {
    background-color:aqua;
    padding: 1rem;
    flex-grow: 1;
    text-align: center;
}
.txt {
    font-size: 2rem;
    line-height: 3rem;
}