Html 引导3模态位置

我在将模态窗口定位到窗口中心时遇到问题。它在右边显示位置。我遗漏了什么，这似乎是我无法解决的CSS问题？谢谢你的建议

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title">I am a modal window, and no one can beat me!</h4>
      </div>
      <div class="modal-body">

        <!-- Columns within the modal window -->
        <div class="row">
          <div class="col-md-3">
            <img src="images/icons/dynamite.svg" alt="" />
          </div>

          <div class="col-md-9">
            <p class="lead">This is a test modal!</p>
          </div>
        </div>

        <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Voluptatibus, omnis, sunt illum et minima facilis rerum eaque ullam quasi aperiam soluta beatae vero atque pariatur ab ad temporibus accusamus necessitatibus.</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-danger" data-dismiss="modal">Close Window</button>
      </div>
    </div>
    <!-- /.modal-content -->
  </div>
  <!-- /.modal-dialog -->
</div>

如果要在中心上设置窗口，则必须使用以下样式：

.modal-dialog{
   position: absolute;
   left: 50%;
   /* now you must set a margin left under zero - value is a half width your window */
   margin-left: -312px;
   /* this same situation is with height - example */
   height: 500px;
   top: 50%;
   margin-top: -250px;
} 

---

杰克·史密斯：你真的是《大爆炸》中的谢尔顿吗？@Hakam Fostok，我想在某个元素下设置对话位置，设置对话窗口的大小，怎么做？

@media screen and (min-width: 768px) {
    .modal-dialog {
        left: 50%;
        right: auto;
        width: 624px;
    }
}

.modal-dialog{
   position: absolute;
   left: 50%;
   /* now you must set a margin left under zero - value is a half width your window */
   margin-left: -312px;
   /* this same situation is with height - example */
   height: 500px;
   top: 50%;
   margin-top: -250px;
} 

.modal-sss-dialog {
    position: absolute;
    margin-left: 200px;
    height: 500px;
    top: 50%;
    margin-top: -250px;
}