Css For Gallery Exclusive Mode Children

I'm wondering if there's a way to inject a small amount of css in either the home page of a Gallery Exclusive Mode gallery, or in the subgalleries?  There doesn't seem to be a html id or class to distinguish parent page from child, so I'm hoping that there are different php pages that create them. 

Alternatively, is there a way I could add class="subpage" to the Body element of subpages across the site?

I'm on 5.2.5, but this has been the way the code is formed since the beginning.

You can add CSS to the embed video or code option, but there isn't anything that separates whether they are viewing the main gallery or a sub gallery.