I find most of the themes do not include a "return to the main site. For now, zp is a big part of my site, but not the main site. Any links leading to it have to be opened into another window because there is no return link to the site entrance. I know this is not true of all the themes that are supported or included but it now seems to be most of the themes.

So, what exactly is your point here? Some themes do this and some dont? It is certainly easy to provide if wanted.

We actually consider Zenphoto as a CMS to run the entire site so most themes don't use that function. As with your other topic regarding dates it always comes down to theming.