opacity ile oyna ama yanlış kodlama var. hem css verip hem style vermek yanlış
Edit : Bir web sitesinde kullandığım css ;

margin: 0;
 font-size: 14px;
 color: #333;
    font-family: "Roboto",sans-serif;
    background: rgba(233,233,233,.3607843137);
    font-weight: 300;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    background-image: url(../../images/sp_main_bg.png);
    background-size: cover;
    -webkit-background-size: cover;
    background-attachment: scroll;
    background-position: center center;
    background-repeat: no-repeat;