Adding HTML above Content.

Ello,

I’m trying to add HTML above my content, in the index.php file.
It works, but it aligns it to the very left of the page.

When I try to center it, it goes underneath the right sidebar.
I don’t want to offset it with pxs with css, because it offsets my mobile site, which works just fine when I use css to align it in the center.

Anyway I can get it centered with my content, and so it won’t interfere with the right sidebar?

I’m putting my code under
<div id="content" class="site-content" role="main">

PS. I dont want it in header.php because I only want it on the homepage, where all of my posts are.
Thanks.