Add HTML code outside content area on every page load I am trying to add HTML code outside the content area on every page load with the use of:

function us_page_build(&amp;$page) {
  $page['content'][] = array(
    '#markup' =&gt; '&lt;div class="fullscreen"&gt;&lt;p&gt;Here is my div content!!&lt;/p&gt;&lt;/div&gt;',
  );
}


However, in this way I am placing the element within the content attribute. Is there a way to set it outside, having as parent only the html element? 
I am aware I can paste code into the template files, but then if I switch to another template(which I will) I will need to re-enter it.
Content is a region in your theme. Site content goes into regions. So, typically you can only put things in defined regions such as Header, Footer, Left Sidebar, Content.

You have slight flexibility with #prefix and #suffix render array attributes. You either have to edit the html.tpl.php file, or define your own region within a theme. Or you can supply your own hook_theme() implementation to hint when using your own html.tpl.php template file.