html, body { height: 100%; overflow: hidden; }
body {
    font-family: "Verdana", "Tahoma", "Lucida Grande", "Microsoft YaHei", "Hiragino Sans GB", sans-serif;
    background: url(../images/loginbg.jpg) no-repeat center center fixed;
    background-size: cover;
}
.form-control{height:37px;}/*输入框的高度*/
.main_box {
    position:absolute;/*位置 :绝对值*/
    top:50%;
    left:50%;
    margin:-200px 0 0 -180px;/*外边距 上 右 下 左*/
    padding:15px 30px 5px 30px;/*内边距 上 右 下 左*/
    width:360px;/*宽度*/
    height:420px;/*高度*/
    min-width:320px;/*最小宽度*/
    background:#FAFAFA;/*背景色*/
    background:rgba(255,255,255,0.4);/*渐变: R G B A(0-1之间取值)*/
    box-shadow: 1px 5px 8px #888888;
    border-radius:10px;/*倒角*/
}
.login_logo {margin-bottom: 20px; height: 45px; text-align: center;}
.login_logo img {height: 60px;}
.login_msg {text-align: center; font-size: 20px;}
.input-group >.input-group-addon.code{padding:0;}
.captcha_img{cursor:pointer;}
.main_box .logo img{height:60px;}
@media (min-width: 768px) {
    .main_box {margin-left:-240px; padding:15px 30px 5px 30px; width:460px;}
    .main_box .logo img{height:45px;}
}