header{
    display: block;
    text-align: center; 
    width: 100%;
    margin: 0 auto;     /*上下边界为0，左右自动居中*/
}
header h1{
    font-family: Arial;
    font-size: 40px;    
    font-weight: bold;  /*粗体*/

}   
header #newGameButton{
    display: block;
    margin: 20px auto;
    width: 100px;
    border-radius: 5px;
    padding: 10px 10px; /*上下内边距为10px，便于文字与边框有一定的距离*/

    color: white;
    font-family: Arial;
    text-decoration: none;  /*去掉链接下划线*/

    background-color:darkgreen ;   
}

/*鼠标移动过去样式的改变*/
header #newGameButton:hover{
    background-color:forestgreen;
}

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

#grid-container{
    width: 460px;
    height: 460px;
    margin: 50px auto;
    padding: 20px;
    background-color:steelblue;
    border-radius: 10px;
    
    position: relative;
}

.grid-cell{
    width: 100px;
    height: 100px;
    background-color:cornflowerblue;
    border-radius: 6px;
    position: absolute;
}

.number-cell{
    border-radius: 6px;

    font-family: Arial;
    font-weight: bold;
    font-size: 60px;
    line-height: 100px;

    text-align: center;
    position: absolute;
}