Search the community, it's been asked and answered
The Community site theme is available at https://github.com/Elgg/community. But it wouldn't work to use it directly on your site as it's customized specifically for the community site.
The site menu isn't in the header anyway here on the site. It's just that the header is empty and therefore the menu appears to be on top directly below the topbar. A blank header can be achieved by overriding the header view like done in https://github.com/Elgg/community/blob/master/views/default/page/elements/header.php.
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