header {
    display: block; /*块级元素，会换行*/
    margin: 0 auto;
    width: 500px;
    text-align: center;
}

header h1 {
    font-family: Arial;
    font-size: 60px;
    font-weight: bold;
}

header #newgamebutton {
    display: block;
    margin: 20px auto;

    width: 100px;
    padding: 10px 10px;
    background-color: #8f7a66;

    font-family: Arial;
    color: white;

    border-radius: 10px;

    text-decoration: none; /*文本修饰：可以上划线、下划线、删除线等*/

}

header #newgamebutton:hover {
    /*hover是伪类，鼠标滑过效果*/
    background-color: #9f8b77;
}

header p {
    font-family: Arial;
    font-size: 25px;
    margin: 20px auto;
}

#grid-container {
    width: 460px;
    height: 460px;

    padding: 20px;
    border-radius: 10px;
    margin: 50px auto;

    background-color: #bbada0;

    position: relative; /*根据元素正常位置进行相对定位，absolute为绝对定位*/
}

.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: 20px;
    line-height: 100px; /*一个格子height为100px，将行高设置为100px*/
    text-align: center;

    position: absolute;
}