background-color:#ff6600
background-color:rgb(0,0,0)黑色
background-color:rgba(0,0,0,0.2)
a是指透明度 范围0-1
第一张图不平铺，第二张图平铺
background-image:url("xxx/img/logo.png"),url("xxx/img/logo2.png");
background-repeat:no-repeat,repeat;
