silverstripe-webpack/app/client/src/scss/layout/navigation.scss

#Header {
    .element--nav {
        padding: 0;
    }
}

#Navigation {
    padding: 0;

    .navbar-nav {
        padding: 0;
        width: 100%;
        align-items: stretch;
        justify-content: stretch;
    }

    .nav {
        &-item {
            flex: 1 1;
        }

        &-link {
            background: none;
            align-items: center;
            justify-content: center;
            width: 100%;

            &.active {}

            &.section {}

            &[aria-expanded="true"],
            &:hover,
            &:focus {}
        }
    }
}

#PageBreadcumbs {
  position: relative;
  z-index: 2;
}