背景图片设置透明度而不改变内容

<div class="nav">内容</div>

.nav{
  width: 100%;
  height: 3.55rem;
  &::before{
    content: "";  
    background: url('../../../assets/img/service.png') no-repeat center top;
    background-size: contain;
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    z-index: -1; /* -1 可以当背景 */ 
    opacity: 0.6;
  }
}