The Avada theme’s hero image floats behind the .fusion-tb-header on desktop, but on mobile this floating behaviour breaks down and exposes a solid black header background instead of the hero image, since the header treats mobile differently from desktop by default.
Fix
Rather than relying on Avada’s Dynamic CSS pipeline (found to be unreliable for this scoped fix), a must-use plugin was added at wp-content/mu-plugins/mobile-header-fix.php, injecting scoped CSS at max-width: 800px. The same mu-plugin also handles: hiding the desktop logo on mobile, correcting hamburger menu icon visibility, and a margin-top / padding-top fix removing a white gap that appeared above .hero-banner-contain on mobile.