.index {
  margin: 0 auto;
  margin-top: 300px;
  display: flex;
  flex-direction: column;
  align-items: center;
  position: relative;
  width: 600px;
  border: 5px solid transparent;
  border-radius: 50px;
  background-clip: padding-box, border-box;
  background-origin: padding-box, border-box;
  background-color: black;
  /* 不透明度 */
  opacity: 50%;
  /* 居于幕布上层 */
  z-index: 2;
}

.title {
  font-size: 30px;
  font-weight: normal;
  position: relative;
  color: white;
}

.teacher_btn,
.student_btn {
  background-color: #fff;
  color: #333;
  padding: 15px 32px;
  font-size: 20px;
  cursor: pointer;
  margin: 15px 10px;
  border-radius: 20px;
  box-shadow: 0px 2px 5px rgba(0, 0, 0, 0.3);
}

.teacher_btn:hover,
.student_btn:hover {
  box-shadow: 2px 2px 15px 2px rgb(190, 225, 255);
  background-color: transparent;
  color: white;
  animation: btn_motion 0.5s;
}

@keyframes btn_motion {
  from {
    background-color: rgba(255, 255, 255, 0.582);
    box-shadow: 0px 0px 15px 2px rgb(190, 225, 255);
  }

  to {
    background-color: transparent;
    color: white;
    box-shadow: 2px 2px 15px 2px rgb(190, 225, 255);
  }
}