No, I don't. I don't think ThemeDesigner has a border option. Perhaps you meant background image? I am not sure when they added the CSS but ThemeDesigner now has a full-page option. When combined with the cover option, it should fill the entire page. Either way, could you provide a link to the wiki? Especially if the appearance has been modified with site CSS already, I am definitely going to have to see what the specific situation is.
> Our admins ... when the border is applied through theme designer... .
Sorry, early morning brain fog. I meant background.
It covers most of the page, but not the part at the bottom. I've attached two images, the first is with safe mode, i.e. no CSS on. This is the theme designer image. A red arrow points towards what I mean:
And the second image is with custom CSS, hopefully it's a bit more clear now: