/* Add custom CSS styles below */
.promotion-title, .s-product-card-promotion-title {
    position: absolute;
    top: 1rem;
    z-index: 10;
    background-color: rgba(28, 45, 68, 0.8);
    color: #fff;
}
.promotion-title, .s-product-card-promotion-title {
    position: absolute;
    top: 1rem;
    z-index: 10;
    background-color: rgba(28, 45, 68, 0.4); /* شفافية 40% */
    color: #fff;
}

 <div id="mainnav" class="main-nav-container shadow-default bg-white" style="height: 56px; background-color: #1c2d44;">

#mainnav {
    background-color: #1c2d44 !important;
}

<a href="https://almesbahi.sa/category/zZmjKp" aria-label="عروض نهاية الموسم" target="_self" style="text-decoration: none;">
    <span style="color: red; font-weight: bold; animation: blink 1s infinite;">عروض نهاية الموسم</span>
</a>

<style>
    @keyframes blink {
        0% { opacity: 1; }
        50% { opacity: 0; }
        100% { opacity: 1; }
    }
</style>