style.css

header{
    display: block;
    margin: 0 auto;
    width:100%;
    text-align:center;
}
header h1{
    font-family: Arial;
    font-size: 60px;
    font-weight: bolder
    margin:0 auto;
}
header #new_game_button{
    display:block;
    margin:0px auto
    width:100px;   
    padding:10px 10px;
    background-color: #8f7a66;
    font-family: Arial;
    color:white;
    border-radius: 10px;
    text-decoration: none;
}
header p{
    display: block;
    font-family: Arial;
    margin:5px auto
    padding:5px 5px;
    color:black;
}
#grid_container{
    width:460px;
    height:460px;
    padding:20px;
    margin:0px auto;
    background-color: #bbada0;
    border-radius: 10px;
    position: relative;
}
.grid_cell{
    width:100px;
    height:100px;
    border-radius: 6px;
    background-color: #ccc0b3;
    position: absolute;
}
.number_cell{
    border-radius: 6px;
    font-family: Arial;
    font-weight: bold;
    font-size: 60px;
    line-height: 100px;
    text-align: center;
    position: absolute;
}