src/themes/dspace/app/header-nav-wrapper/header-navbar-wrapper.component.scss
:host {
  --ds-header-navbar-border-bottom-style: solid var(--ds-header-navbar-border-bottom-height) var(--ds-header-navbar-border-bottom-color);
  --ds-expandable-navbar-border-top-style: solid var(--ds-expandable-navbar-border-top-height) var(--ds-expandable-navbar-border-top-color);
  --ds-expandable-navbar-height: calc(100vh - var(--ds-header-height));

  div#header-navbar-wrapper {
    // The header-navbar-wrapper should not have a z-index, otherwise it would cover the media viewer despite its higher z-index
    position: relative; // required by the mobile collapsible navbar
    border-bottom: var(--ds-header-navbar-border-bottom-style); // gets covered by mobile navbar wrapper, when open

    div#mobile-navbar-wrapper {
      width: 100%;
      background-color: var(--ds-expandable-navbar-bg);
      position: absolute;
      z-index: var(--ds-nav-z-index);
      overflow: hidden;

      nav#collapsible-mobile-navbar {
        // Following parameters are changed by slideMobileNav animation
        min-height: var(--ds-expandable-navbar-height);
        height: auto;
        border-bottom: var(--ds-header-navbar-border-bottom-style);
      }
    }
  }

  /* MENU ITEMS */

  ::ng-deep {
    .ds-menu-item, .ds-menu-toggler-wrapper {
      text-decoration: none;
    }

    nav#desktop-navbar { // in header component
      #main-site-navigation {
        /* Desktop menu */

        .ds-menu-item-wrapper, .ds-menu-item, .ds-menu-toggler-wrapper {
          // fill navbar height (required by dropdown menus) and center content
          display: flex;
          align-items: center;
          height: 100%;
        }
        .ds-menu-item {
          // define here the style for top-level navbar menu items
          padding: var(--ds-navbar-item-vertical-padding) var(--ds-navbar-item-horizontal-padding);
        }
        .ds-menu-item, .ds-menu-toggler-wrapper {
          color: var(--ds-navbar-link-color) !important;
          &:hover, &:focus {
            color: var(--ds-navbar-link-color-hover) !important;
          }
        }

        /* desktop submenu */

        .dropdown-menu {
          .ds-menu-item {
            // define here the style for second-level navbar menu items
            padding: var(--ds-navbar-dropdown-item-vertical-padding) var(--ds-navbar-dropdown-item-horizontal-padding);
          }
        }

      }
    }

    nav#collapsible-mobile-navbar { // in header-navbar-wrapper component

      border-top: var(--ds-expandable-navbar-border-top-style);
      padding-top: var(--ds-expandable-navbar-padding-top);

      #main-site-navigation {
        .ds-menu-item {
          padding: var(--ds-expandable-navbar-item-vertical-padding) 0;
        }
      }

    }
  }
}
