index.css

.multi-select-panel {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 480px;
  padding: 20px;
  transform: translate(-50%,-50%);
  background: #fff;
  box-shadow: 0 0 8px rgba(0,0,0,0.1);
  border-radius: 6px;
  overflow: hidden;
}
.multi-select-panel .middle-controllers {
  float: left;
  width: 140px;
  text-align: center;
}

.multi-select-panel .middle-controllers button {
  display: block;
  margin: 0 auto;
  margin-bottom: 20px;
}

.multi-select-panel .left-selects, .right-selects {
  float: left;
  width: 170px;
  height: 200px;
}
.multi-select-panel option {
  padding: 4px;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
.multi-select-panel select {
  width: 170px;
  height: 200px;
  overflow-y: auto;
  font-size: 12px;
  color: #666;
  outline: none;
  border: 1px solid #aaa;
  border-radius: 3px;
}

.multi-select-panel .confirm-selects {
  padding-top: 30px;
  text-align: center;
  clear: both;
}
.multi-select-panel .confirm-selects button {
  margin: 0 24px;
  padding: 0 20px
}