Reply To: background image on a page?

It depends, which part of the page? The entire page? or just a specific area? Also, are you trying to do this throughout the entire site or just that specific page?

Sitewide – you could just add something to the body on line 152

For that specific page you’d need to create code for that particular page ID/Class, which is page-id-1152 and then add that in front of .site-inner. Here is the site-inner code which wraps the content area (on line 552 in your css) –

.site-inner {
    background: #f5f5f5;
    clear: both;
    padding-top: 40px;
    padding-top: 2.5rem;
}

you would need to add separate code like this – .page-id-1152 .site-inner