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

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

#Navigation {
    .navbar {
        padding: 0;
    }

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

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

        &-link {
        }
    }
}

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