I would recommend always having a nice theme/design on any site that anyone is developing, but as some of the above have mentioned, the all in one seo plugin is great for this!
Go and look for a right one on some theme providing sites. It is better for you to pick up one from various themes.
In wordpress theme is not so valuable for SEO instead you choose a theme which is related to your niche and then mostly wordpress need cleanup. Cleanup include removing extra CSS, remove unnecessary PHP and JQuery. When you done then install best plugin to make it useful for SEO.
Themes are meant for users, not Googlebots, spiders or crawlers. A clean and simple theme will be more than enough to make your Wordpress blog look legit.
IMO use that theme which can get loaded earlier and faster because it doesn't worth of being on even #1 rank if your site is loading very slowly. You'll only achieve higher bounce rate, so choose a theme which could be loaded earlier and faster.
I agree that on page SEO is the first step but there are some themes that work well and compliment the on page SEO content. I have used Thesis and ThemeMX with great results...we're talking first page rankings in conjunction with some link building in a few short weeks.
I don't think there is an exact theme which is good for seo lol Search engines only like nice and well coded websites, so make sure the theme is well coded and that's it!