@ipunkt: Go to => Appearance => Customize => Additional CSS and add the following CSS Code.
.site-content-contain {
background-image:url("https://www.w3schools.com/css/img_lights.jpg");
background-repeat:no-repeat;
background-size:cover;
background-position:center;
}
.site-content .wrapper {
background:none !important;
}
.widget-area .widget {
background:rgba(255,255,255,0.4) !important;
}
.site-main .hentry, .widget {
background:rgba(255,255,255,0.4) !important;
}
Replace the url in background-image with your image URL.
Let me know if this is what you want!
Kind Regards,
Skandha