Duplicate layout without plagiarizing?

My question is at the intersection of ethics and practicality. I'm creating a Wordpress theme for a friend and he has shown an example of a page he wants to emulate. The page has a custom WP theme, so he can't just purchase something similar out-of-the-box.

Thus, I'm trying to re-create the same kind of layout, but changing the look & feel enough to not have it resemble to original target page. Essentially, trying to duplicate the layout (nothing too unique about it, just works well aesthetically) without blatantly copying someone else's work. I think it's a generic enough div layout, and I'm changing other style elements (fonts, color scheme, etc).

How do you all go about this kind of task? Very subjective question, I know, but how much do you think is OK to copy from the other site's CSS before putting my own around it? Do all WP theme developers re-invent the wheel from scratch for every layout? Or is there an acceptable starting point where you can work from existing stylesheets, so long as you're not overtly copying someone's site.

Thanks!