This should be in the theme options. It doesn’t work in the custom CSS on the header.

/* -- START Transparant background for full width header -- */
.et_pb_fullwidth_header_container .header-content-container {
background-color: rgba(255,255,255,0.7) !important;
}
/* -- END Transparant background for full width header -- */Code language: CSS (css)

Or

/* -- START Transparant background for full width header -- */#main-header::after {
content: "";
background-image: url('yourimagegoeshere.png');
background-size:cover;
opacity: 0.5;
top: 0;
left: 0;
bottom: 0;
right: 0;
position: absolute;
z-index: -1;
}
/* -- END Transparant background for full width header -- */Code language: CSS (css)