Found In: Confluence Administration -> Look And Feel -> Custom HTML
At the Custom HTML level You can add global HTML that will be included through the entire site.
This can be links to static CSS files (on the server filesystem or added as an attachment on a page)
This is the only place (GUI wise) that You can add to the <HEAD> tag.
Its a logical place to place things like extra JavaScripts and global styling
Scopes for Confluence CSS Styling