ese-website/_includes/header.css

header {
  display: flex;
  position: sticky;
  top: 0;
  min-height: 9rem;
  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 {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

#header-logo img {
  max-height: 60px;
  display: block;
}

#mainNav ul {
  margin: 0;
  padding: 0;
}

#mainNav li {
  display: inline-block;
  margin-left: 1.9rem;
}

#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,
#mainNav .nav-active a::after {
  opacity: 1;
}