body{
    flex-direction: column;
}
main{
    flex: 1;
    display: flex;
    justify-content: center;  /* 水平方向中央 */
    align-items: center;      /* 垂直方向中央 */
    padding-top: 0px;
    padding-bottom: 20px;
    text-align: center;
}
input[type="submit"]{
    margin-top: 20px;
    padding-top: 5px;
    padding-left: 10px;
    padding-right: 10px;
    padding-bottom: 5px;
    margin-left: 0px;
    border-radius: 10px;
    border: 1px solid #bbbbbb;
}

li{
    list-style-type: none;
}