I never thought the design would impact SEO but I've had an odd experience recently.
I put up 4 sites using the same slightly modified template & all 4 of them were nowhere to be seen in Google.
2 of them did initially show up around page 1 & 2 then suddenly dropped like a brick to about page 30.
I did a bit of investigating, changed the <!DOCTYPE html PUBLIC bit of the code, and hey presto 2 of them have re-appeared.
Just waiting for the other 2 to be crawled & we'll see what happens.