style-global-as-background-opacity.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;    
  }
  body {
    background: transparent;    
    overflow-x: hidden;   
    margin: 0;
    padding: 0; 
    height: 100%;
    
    // hack background opacity
    &::before {
      position: absolute;
      top: 0;
      left: 0;
      bottom: 0;
      right: 0;
      height: 100%;
      background: url("/path/to/background-image/myimage.png")
        no-repeat center center;
      background-size: cover;
      opacity: 0.1;
      content: "";
      z-index: -1;
    }
  }
  html {
    font-size: 16px;
  }