#container {
    background-color: #959595; 
    margin: 0 auto;
    width: 500px;
    height: 500px;
    position: relative;
}

.square {
    position: absolute;
    border: 10px solid black;
}

.red {
    background-color: red;
}
.blue {
    background-color: blue;
}
.yellow {
    background-color: yellow;
}
.white {
    background-color: white;
}

.box {
    position: absolute;
    border: 5px solid black;
    border-radius:40%;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 16px;
    text-align: center;
}