message: [improvement] Stepper: use to set button size
diff: &__plus {
  position: relative;
  box-sizing: border-box;
  - width: 28px;
  - height: 28px;
  + width: @stepper-input-height;
  + height: @stepper-input-height;
  margin: 0;
  padding: 5px;
  color: @stepper-button-icon-color;
  