给 background-image 设置透明度！

admin

div{
    position: relative;        
}
div:after{
    content: '';
    display: block;
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    background: url(...);
    opacity: 0.5;
}

admin

渐变色：
background-image: linear-gradient(rgba(0, 0, 0, 0.9), rgba(0, 0, 0, 0.1)), url(../img/glow.jpg);

变黑底：
background-image: linear-gradient(rgba(0, 0, 0, 0.8), rgba(0, 0, 0, 0.8)), url(../img/glow.jpg);

变白底：
background-image: linear-gradient(rgba(255, 255, 255, 0.9), rgba(255, 255, 255, 0.9)), url(../img/glow.jpg);