Add margin-top when user is logged in.

[StevenDufresne]

Describe the bug

We don't include any top margin to account for the admin bar on WordCamp sites and therefore the admin bar overlaps with the header. You can see that at https://asia.wordcamp.org/2025/.

I was thinking we could do something like:

/**
 * Render CSS to adjust the top margin when the admin bar is present for logged-in users.
 */
function wcorg_admin_header_tweak_css() {
	if ( is_user_logged_in() ) {
		$admin_bar_height = 'var(--wp-admin--admin-bar--height, 0)';
		?>
		<style type="text/css">
			html {
				margin-top: <?php echo esc_attr( $admin_bar_height ); ?>;
			}
		</style>
		<?php
	}
}

add_action( 'wp_head', 'wcorg_admin_header_tweak_css' );

Note

It seems like some WordCamp sites have already shipped fixes so whatever we do here, we'll need to reach out to those organizers ahead of time.

@ryelle What are you thoughts about adding this to https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/mu-plugins/3-helpers-misc.php