ese-website/_includes/header.njk

<header>
  <div class="header-container">
    <a href="{{ "/" | locale_url }}" title="home">ESE Logo</a>
    <nav id="mainNav">
      {{ collections.mainNav | pageLang | eleventyNavigation | eleventyNavigationToHtml | safe }}
    </nav>
  </div>
</header>

{% css %}
header {
  display: flex;
  position: sticky;
  top: 0;
  min-height: 6.4rem;
  padding: 2rem 0;
  backdrop-filter: blur(5px);
  background-color: rgba(255,255,255,.92);
  box-shadow: 0 3px 15px hsla(0,0%,39.2%,.12);
}
.header-container {
  width: 100%;
  max-width: var(--page-width);
  margin-left: auto;
  margin-right: auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
}
#mainNav ul {
  margin: 0;
  padding: 0;
}
#mainNav li {
  display: inline;
  margin-left: 1rem;
}
#mainNav a {
  position: relative;
  font-size: 16px;
  font-family: Aileron;
  font-weight: 500;
  line-height: 2;
  text-transform: uppercase;
  color: var(--color-text);
  text-decoration: none;
}
#mainNav a::after {
  content: "";
  position: absolute;
  background: var(--color-primary);
  opacity: 0;
  bottom: -4px;
  left: 0;
  right: 0;
  height: 2px;
  transition: opacity .3s ease-out;
}
#mainNav a:hover::after {
  opacity: 1;
}
{% endcss %}