/* 右上角返回按钮 */
.iconfont {
  z-index: 2;
  position: fixed;
  right: 40px;
  top: 20px;
  width: 100px;
  font-size: 100px;
}

/* 左上角导航栏 */
.nav {
  display: flex;
  position: fixed;
  top: 80px;
  left: 50px;
}

.nav_box {
  margin: 8px;
}

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

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

@keyframes btn_mation {
  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);
  }
}

/* 黑板做提示 */
.board {
  left: calc((500px - 200px)/2);
  position: fixed;
  width: 250px;
  height: 220px;
  background-color: black;
  /* 不透明度 */
  opacity: 50%;
  text-align: center;
  color: white;
  font-size: 20px;
  line-height: 30px;
  padding: 25px;
}

::-webkit-input-placeholder {
  /* WebKit browsers */
  color: #999;
  font-size: 26px;
  font-style: italic;
}

::-moz-placeholder {
  /* Mozilla Firefox 19+ */
  color: #999;
  font-size: 26px;
  font-style: italic;
}

:-ms-input-placeholder {
  /* Internet Explorer 10+ */
  color: #999;
  font-size: 26px;
  font-style: italic;
}