div居中

.bgform {
    top:50%;
    left:50%;
    position: absolute;
    /*right: 10px;*/
    /*bottom: 150px;*/
    background: rgba(255, 255, 255, 0.20);
    border: 1px solid rgba(183, 255, 255, 0.27);
    border-radius: 2px;
    width: 350px;
    height: 350px;
    padding: 5px auto;
    margin:-175px 0 0 -175px;
}