Anfängerfehler!
Die Stelle befindet sich in der LESS:
…\engine\Shopware\Plugins\Default\Frontend\AdvancedMenu\Views\frontend_public\src\less\advanced-menu.less
dann…
.navigation-main .navigation--list .navigation--entry.is--hovered { .border-radius-multi(3px, 3px); .tap-highlight-color(rgba(0, 0, 0, 0)); background: @adv-menu-header-bg; .navigation--link { background: @adv-menu-header-bg; color: @adv-menu-header-text-color; } }