I am using the Moesia theme. Because it is part of a multisite, I am not sure how to access the header.php file for just this site.
In multisite all themes serve all sites, so if you edit a theme it will affect to all sites.
The solution is you create a theme dedicated to one site. Ex; theme 1 for site 1. Activate theme 1 for site 1 and then edit theme 1 header.php file and it will affect to only site 1.