校园网开发问题

老师，我真的想进办法让这个下边框居中了

怎么让它到o啊

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>校园网首页开发lding</title>
    <script src="js/js.js"></script>
    <link rel="stylesheet" href="css/css.css">
</head>
<body>
    <div class="header">
        <div class="logo">
            <img src="image/logo.png">
        </div>
        <div class="function">
            <ul>
                <li id="f1"><a href="#" onmouseover="show('f1')" onmouseleave="show1('f1')">HOME</a></li>
                <li id="f2"><a href="#" onmouseover="show('f2')" onmouseleave="show1('f2')">ABOUT</a></li>
                <li id="f3"><a href="#"onmouseover="show('f3')" onmouseleave="show1('f3')">GALLERY</a></li>
                <li id="f4"><a href="#"onmouseover="show('f4')" onmouseleave="show1('f4')">FACULTY</a></li>
                <li id="f5"><a href="#"onmouseover="show('f5')" onmouseleave="show1('f5')">EVENTS</a></li>
                <li id="f6"><a href="#"onmouseover="show('f6')" onmouseleave="show1('f6')">CONTACT</a></li>
            </ul>
        </div>
    </div>
    <div class="banner">
        <div class="banner-img">
            <img src="image/banner3.jpg">
        </div>
        <div class="textdiv">
            <ul>
                <li><input type="text" id="text1" class="text" value="your Name"></li>
                <li><input type="text" id="text2" class="text" value="your Phone"></li>
                <li><input type="text" id="text3" class="text" value="your Email"></li>
                <li><textarea rows="10" cols="30" id="text4">
                                                                                                               Write Your Comment Here</textarea></li>
                <li><button id="btn" type="submit" value="SEND MESSAGE">SEND MESSAGE</button></li>

            </ul>
        </div>

    </div>
    <div class="about">
        <div class="about-h"><h2>ABOUT</h2></div>
    </div>
    <div class="gallery">

    </div>
    <div class="footer">

    </div>
</body>
</html>

css

*{
    margin: 0px;
    padding: 0px;
}
.header{
    width: 100%;
    height: 60px;
    background-color: #07cbc9;
}
body{
    font-family:  Microsoft YaHei UI;
    font-size: 14px;
}
.header .logo{

    float: left;
    position: relative;
    margin-top: 6px;
    margin-left: 100px;
}
.header .function ul{
    float: right;
    margin-right: 60px;
}
.header a{
    text-decoration: none;
    color: white;
    font-size: 10px;
    font-weight: bold;
}

.header .function ul li{
    float: left;
    margin-right: 5px;
    line-height: 59px;
    list-style: none;
    background-color:#07cbc9;
    padding-left: 10px;
    padding-right: 10px;
}

.banner{
    margin-top: 0px;
    width: 100%;
    height: 400px;
}
.banner .banner-img img{
    width:100%;
    height: 500px;
    z-index: 10;
    float: left;
    position: relative;
    filter:contrast(80%);
    filter:brightness(70%);
    opacity: 0.9;
}

.banner .textdiv ul li .text{
    width: 400px;
    height: 30px;
    list-style: none;
    background-color: transparent;
    /*border-width: medium;*/
    border: 1px solid #ccc;
    outline: none;
    color: darkgray;
    font-family:  Microsoft YaHei UI;
    font-size: 10px;
}
.banner .textdiv ul li .text:hover {
    border: 1px solid #07cbc9;
}
.banner .textdiv ul li #text4{
    height: 100px;
    width: 400px;
    text-align: left;
    background-color: transparent;
    vertical-align:top;
    border: 1px solid darkgray;
    outline: none;
    color: darkgray;
    font-family:  Microsoft YaHei UI;
    font-size: 10px;

}
.banner .textdiv ul li #text4:hover{
    border: 1px solid #07cbc9;
}
.banner .textdiv ul li{
    margin-bottom: 15px;

}
.banner .textdiv{
    width: 100%;
    height: 400px;
    z-index: 20;
    margin-top: 50px;
    /*background-size:contain;*/
    /*background: url(../image/banner3.jpg);*/
    /*background-repeat: no-repeat;*/
    float: left;
    position: absolute;
    text-align: center;

}
.banner .textdiv ul{
    margin-top: 60px;
}
.banner .textdiv #btn{
    height: 30px;
    width: 130px;
    background-color: transparent;
    border: 1px solid  darkgray;
    outline: none;
    color: darkgray;
    font-family:  Microsoft YaHei UI;
    font-size: 15px;
}
.banner .textdiv #btn:hover{
    border: 1px solid #07cbc9;
}
body{
    overflow: auto;
}
.about{
    margin: 150px auto;
}
.about .about-h{

    font-family:  Microsoft YaHei UI;
    font-size: 25px;
    font-weight: bold;
    margin: 150px 45%;
    text-align: center;
    border-bottom: #07cbc9 1px solid ;
    width: 30px;

    /*width: 30px;*/
}