styles.css

@import url('https://fonts.googleapis.com/css2?family=Cabin+Sketch&display=swap');

:root {
    --cell-count: 16;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Cabin Sketch', cursive;
}

body {
    background-color: white;
    margin: 0 auto;
    text-align: center;
}

header {
    background-color: white;
}

header > h1 {
    text-align: center;
    font-size: 80px;
    margin: 20px auto;
    border: 2px solid #ECA3A3;
    width: 600px;
    padding: 10px;
}

#instruction {
    text-align: center;
    margin: 10px;
}

#box-input {
    text-align: center;
    margin: 10px;
}

#button-inputs {
    text-align: center;
    margin: 10px;
}

.buttons{
    margin: 0 10px;
}

.buttons:hover {
    cursor: pointer;
    background: #ECA3A3;
}

input {
    width: 50px;    
}

#borderContainer {
    width: 980px;
    height: 980px;
    border: 10px solid red;
    border-radius: 5px;
    text-align:center;
}

#container {
    width: 960px;
    height: 960px;
    /*border: 0px solid red;*/
    margin: 0 auto ;
    border-radius: 2%;
    display: flex;
    flex-wrap: wrap;
}

.cell {
    margin: 0;
    padding: 0;
    border: 1px solid black;
    height: calc(960px/var(--cell-count));
    width: calc(960px/var(--cell-count));
}