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

#SiteWideMessage {
    text-align: center;

    .alert {
        margin-bottom: 0;

        .btn-close {
            float: right;
            margin-top: -.5rem;
        }
    }
}

#Header {
    position: relative;
    z-index: 9;

    .element {
        margin-top: 0;
    }

    .nav-container {
        position: static;
        display: flex;

        align-items: center;
        justify-content: flex-end;
    }
}