* {
    margin: 0;
    padding: 0;
    background-color: #F0F8FF;
}

#layout {
    display: flex;
    /* 主轴方向为纵向 */
    flex-direction: column;
    width: 80%;
    /* 居中 */
    margin: 0 auto;
}

#top {
    width: 100%;    /* 这里的100%相当于父级 layout 的80%*/
    flex: 0 0 50px;
    margin: 0 auto;
    background-color: yellow;
    /* height: 50px; */
}

#main {
    flex: 0 0 100%;
    display: flex;
    flex-direction: row;   /* 默认，可设可不设 */
}

#main div:first-child {
    flex: 0 0 10%;
    background-color: #F0F8FF;
    list-style-type: none;
    display: flex;
    /* 设置允许换行 */
    flex-wrap: wrap;
    border-left: 1px solid white;
    border-right: 1px solid white;
}

#main div:first-child li {
    flex: 0 0 100%;
    height: 40px;
    /* 设置文字水平居中 */
    line-height: 40px;
    text-align: center;
    border-bottom: 1px solid white;
}

#main div:nth-child(2) {
    flex: 0 0 90%;
    background-color: #F0F8FF;
    list-style-type: none;
    display: flex;
    /* 设置允许换行 */
    flex-wrap: wrap;
    /* 设置间隔 */
    justify-content: space-around;
}


#main div:nth-child(2) li {
    flex: 0 0 30%;
    height: 120px;
    text-align: center;
    border-bottom: 1px solid white;
    background-color: greenyellow;
    margin-top: 10px;
}