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 {
  position: sticky;
  backdrop-filter: blur(5px);
  background-color: #ffffffcc;
  top: 0;
  padding: 2rem 0;
}
.header-container {
  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;
}
{% endcss %}